Pneumonia
Overview
 
Pneumonia is an inflammatory
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

 condition of the lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

—especially affecting the microscopic air sacs (alveoli)—associated with fever
Fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

, chest symptoms, and a lack of air space (consolidation) on a chest X-ray
Chest X-ray
In medicine, a chest radiograph, commonly called a chest X-ray , is a projection radiograph of the chest used to diagnose conditions affecting the chest, its contents, and nearby structures...

. Pneumonia is typically caused by an infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

 but there are a number of other causes. Infectious agents include: bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es, fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

, and parasites.
Typical symptoms include cough
Cough
A cough is a sudden and often repetitively occurring reflex which helps to clear the large breathing passages from secretions, irritants, foreign particles and microbes...

, chest pain
Chest pain
Chest pain may be a symptom of a number of serious conditions and is generally considered a medical emergency. Even though it may be determined that the pain is non-cardiac in origin, this is often a diagnosis of exclusion made after ruling out more serious causes of the pain.-Differential...

, fever
Fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

, and difficulty breathing. Diagnostic
Medical diagnosis
Medical diagnosis refers both to the process of attempting to determine or identify a possible disease or disorder , and to the opinion reached by this process...

 tools include x-rays and examination of the sputum
Sputum
Sputum is mucus that is coughed up from the lower airways. It is usually used for microbiological investigations of respiratory infections....

.
Encyclopedia
Pneumonia is an inflammatory
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

 condition of the lung
Lung
The lung is the essential respiration organ in many air-breathing animals, including most tetrapods, a few fish and a few snails. In mammals and the more complex life forms, the two lungs are located near the backbone on either side of the heart...

—especially affecting the microscopic air sacs (alveoli)—associated with fever
Fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

, chest symptoms, and a lack of air space (consolidation) on a chest X-ray
Chest X-ray
In medicine, a chest radiograph, commonly called a chest X-ray , is a projection radiograph of the chest used to diagnose conditions affecting the chest, its contents, and nearby structures...

. Pneumonia is typically caused by an infection
Infection
An infection is the colonization of a host organism by parasite species. Infecting parasites seek to use the host's resources to reproduce, often resulting in disease...

 but there are a number of other causes. Infectious agents include: bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

, virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es, fungi
Fungus
A fungus is a member of a large group of eukaryotic organisms that includes microorganisms such as yeasts and molds , as well as the more familiar mushrooms. These organisms are classified as a kingdom, Fungi, which is separate from plants, animals, and bacteria...

, and parasites.
Typical symptoms include cough
Cough
A cough is a sudden and often repetitively occurring reflex which helps to clear the large breathing passages from secretions, irritants, foreign particles and microbes...

, chest pain
Chest pain
Chest pain may be a symptom of a number of serious conditions and is generally considered a medical emergency. Even though it may be determined that the pain is non-cardiac in origin, this is often a diagnosis of exclusion made after ruling out more serious causes of the pain.-Differential...

, fever
Fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

, and difficulty breathing. Diagnostic
Medical diagnosis
Medical diagnosis refers both to the process of attempting to determine or identify a possible disease or disorder , and to the opinion reached by this process...

 tools include x-rays and examination of the sputum
Sputum
Sputum is mucus that is coughed up from the lower airways. It is usually used for microbiological investigations of respiratory infections....

. Vaccine
Vaccine
A vaccine is a biological preparation that improves immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism, and is often made from weakened or killed forms of the microbe or its toxins...

s to prevent certain types of pneumonia are available. Treatment depends on the underlying cause with presumed bacterial pneumonia being treated with antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s.
Although pneumonia was regarded by William Osler
William Osler
Sir William Osler, 1st Baronet was a physician. He was one of the "Big Four" founding professors at Johns Hopkins Hospital as the first Professor of Medicine and founder of the Medical Service there. Sir William Osler, 1st Baronet (July 12, 1849 – December 29, 1919) was a physician. He was...

 in the 19th century as "the captain of the men of death", the advent of antibiotic therapy and vaccines in the 20th century have seen radical improvements in survival outcomes. Nevertheless, in the third world, and among the very old, the very young and the chronically ill, pneumonia remains a leading cause of death.

Classification

Pneumonitis
Pneumonitis
Pneumonitis or pulmonitis is a general term that refers to inflammation of lung tissue.Pneumonia is pneumonitis combined with consolidation and exudation...

 refers to lung inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

; pneumonia refers to pneumonitis, usually due to infection but sometimes non infectious, that has the additional feature of pulmonary consolidation. Pneumonia can be classified in several ways. It is most commonly classified by where or how it was acquired (community-acquired, aspiration
Aspiration pneumonia
Aspiration pneumonia is bronchopneumonia that develops due to the entrance of foreign materials into the bronchial tree, usually oral or gastric contents...

, healthcare-associated
Healthcare-associated pneumonia
In medicine, healthcare-associated pneumonia is a category of pneumonia in patients with recent close contact with the health care system....

, hospital-acquired
Hospital-acquired pneumonia
Hospital-acquired pneumonia or nosocomial pneumonia refers to any pneumonia contracted by a patient in a hospital at least 48–72 hours after being admitted. It is usually caused by a bacterial infection, rather than a virus....

, and ventilator-associated pneumonia
Ventilator-associated pneumonia
Ventilator-associated pneumonia is a sub-type of hospital-acquired pneumonia which occurs in people who are receiving mechanical ventilation. VAP is not characterized by the causative agents; rather, as its name implies, definition of VAP is restricted to patients undergoing mechanical...

), but may also be classified by the area of lung affected (lobar pneumonia
Lobar pneumonia
Lobar pneumonia is a form of pneumonia that affects a large and continuous area of the lobe of a lung.It is one of the two anatomic classifications of pneumonia .- Symptoms :...

, bronchial pneumonia and acute interstitial pneumonia), or by the causative organism. Pneumonia in children may additionally be classified based on signs and symptoms as non-severe, severe, or very severe.

Signs and symptoms

People with infectious pneumonia often have a productive cough
Cough
A cough is a sudden and often repetitively occurring reflex which helps to clear the large breathing passages from secretions, irritants, foreign particles and microbes...

, fever
Fever
Fever is a common medical sign characterized by an elevation of temperature above the normal range of due to an increase in the body temperature regulatory set-point. This increase in set-point triggers increased muscle tone and shivering.As a person's temperature increases, there is, in...

 accompanied by shaking chills, shortness of breath, sharp or stabbing chest pain
Chest pain
Chest pain may be a symptom of a number of serious conditions and is generally considered a medical emergency. Even though it may be determined that the pain is non-cardiac in origin, this is often a diagnosis of exclusion made after ruling out more serious causes of the pain.-Differential...

 during deep breaths, confusion, and an increased respiratory rate
Respiratory rate
Respiratory rate is also known by respiration rate, pulmonary ventilation rate, ventilation rate, or breathing frequency is the number of breaths taken within a set amount of time, typically 60 seconds....

. In the elderly, confusion may be the most prominent symptom. The typical symptoms in children under five are fever, cough, and fast or difficult breathing. Fever, however, is not very specific, as it occurs in many other common illnesses, and may be absent in those with severe disease or malnutrition
Malnutrition
Malnutrition is the condition that results from taking an unbalanced diet in which certain nutrients are lacking, in excess , or in the wrong proportions....

. Additionally, a cough is frequently absent in children less than 2 months old. More severe symptoms may include: central cyanosis, decreased thirst, convulsions, persistent vomiting, or a decreased level of consciousness.
Symptoms frequency in pneumonia
Symptom Frequency
Cough
79–91%
Fatigue
90%
Fever
71–75%
Shortness of breath
67–75%
Sputum
60–65%
Chest pain
39–49%

Some causes of pneumonia are associated with classic, but non-specific, clinical characteristics. Pneumonia caused by Legionella
Legionella
Legionella is a pathogenic Gram negative bacterium, including species that cause legionellosis or Legionnaires' disease, most notably L. pneumophila. It may be readily visualized with a silver stain....

may occur with abdominal pain, diarrhea
Diarrhea
Diarrhea , also spelled diarrhoea, is the condition of having three or more loose or liquid bowel movements per day. It is a common cause of death in developing countries and the second most common cause of infant deaths worldwide. The loss of fluids through diarrhea can cause dehydration and...

, or confusion, while pneumonia caused by Streptococcus pneumoniae
Streptococcus pneumoniae
Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, aerotolerant anaerobic member of the genus Streptococcus. A significant human pathogenic bacterium, S...

is associated with rusty colored sputum, and pneumonia caused by Klebsiella
Klebsiella
Klebsiella is a genus of non-motile, Gram-negative, oxidase-negative, rod-shaped bacteria with a prominent polysaccharide-based capsule. It is named after the German microbiologist Edwin Klebs...

may have bloody sputum often described as "currant jelly".

Physical examination
Physical examination
Physical examination or clinical examination is the process by which a doctor investigates the body of a patient for signs of disease. It generally follows the taking of the medical history — an account of the symptoms as experienced by the patient...

 may sometimes reveal low blood pressure
Hypotension
In physiology and medicine, hypotension is abnormally low blood pressure, especially in the arteries of the systemic circulation. It is best understood as a physiologic state, rather than a disease. It is often associated with shock, though not necessarily indicative of it. Hypotension is the...

, a high heart rate
Tachycardia
Tachycardia comes from the Greek words tachys and kardia . Tachycardia typically refers to a heart rate that exceeds the normal range for a resting heart rate...

, or a low oxygen saturation
Oxygenation (medical)
Oxygenation occurs when oxygen molecules enter the tissues of the body. For example, blood is oxygenated in the lungs, where oxygen molecules travel from the air and into the blood...

. Examination of the chest may be normal, but may show decreased chest expansion on the affected side. Harsh breath sounds from the larger airways that are transmitted through the inflamed lung are termed bronchial
Bronchus
A bronchus is a passage of airway in the respiratory tract that conducts air into the lungs. The bronchus branches into smaller tubes, which in turn become bronchioles....

 breathing, and are heard on auscultation
Auscultation
Auscultation is the term for listening to the internal sounds of the body, usually using a stethoscope...

 with a stethoscope
Stethoscope
The stethoscope is an acoustic medical device for auscultation, or listening to the internal sounds of an animal body. It is often used to listen to lung and heart sounds. It is also used to listen to intestines and blood flow in arteries and veins...

. Rales
Rales
Crackles, crepitations, or rales are the clicking, rattling, or crackling noises that may be made by one or both lungs of a human with a respiratory disease during inhalation. They are often heard only with a stethoscope...

 (or crackles) may be heard over the affected area during inspiration
Inhalation
Inhalation is the movement of air from the external environment, through the air ways, and into the alveoli....

. Percussion
Percussion (medicine)
Percussion is a method of tapping on a surface to determine the underlying structure, and is used in clinical examinations to assess the condition of the thorax or abdomen. It is one of the four methods of clinical examination, together with inspection, palpation and auscultation...

 may be dulled over the affected lung, and increased, rather than decreased, vocal resonance
Vocal resonation
Vocal resonation is the process by which the basic product of phonation is enhanced in timbre and/or intensity by the air-filled cavities through which it passes on its way to the outside air...

 distinguishes pneumonia from a pleural effusion
Pleural effusion
Pleural effusion is excess fluid that accumulates between the two pleural layers, the fluid-filled space that surrounds the lungs. Excessive amounts of such fluid can impair breathing by limiting the expansion of the lungs during ventilation.-Pathophysiology:...

. Struggling to breathe, confusion, and blue-tinged skin
Cyanosis
Cyanosis is the appearance of a blue or purple coloration of the skin or mucous membranes due to the tissues near the skin surface being low on oxygen. The onset of cyanosis is 2.5 g/dL of deoxyhemoglobin. The bluish color is more readily apparent in those with high hemoglobin counts than it is...

 are signs of a medical emergency.

Cause

Pneumonia is primarily due to infections, with less common causes including irritants and the unknown. Although more than one hundred strains of micro organisms can cause pneumonia, only a few are responsible for most cases. The most common types of infectious are virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es and bacteria
Bacteria
Bacteria are a large domain of prokaryotic microorganisms. Typically a few micrometres in length, bacteria have a wide range of shapes, ranging from spheres to rods and spirals...

 with it being less commonly due to fungi or parasites. Mixed infections with both viruses and bacterial may occur in up to 45% of infections in children and 15% of infections in adults. A causative agent is not isolated in approximately half of cases despite careful testing. The term pneumonia is sometimes more broadly applied to inflammation
Inflammation
Inflammation is part of the complex biological response of vascular tissues to harmful stimuli, such as pathogens, damaged cells, or irritants. Inflammation is a protective attempt by the organism to remove the injurious stimuli and to initiate the healing process...

 of the lung (for example caused by autoimmune disease
Autoimmune disease
Autoimmune diseases arise from an overactive immune response of the body against substances and tissues normally present in the body. In other words, the body actually attacks its own cells. The immune system mistakes some part of the body as a pathogen and attacks it. This may be restricted to...

, chemical burns or drug reactions), however this is more accurately referred to as pneumonitis
Pneumonitis
Pneumonitis or pulmonitis is a general term that refers to inflammation of lung tissue.Pneumonia is pneumonitis combined with consolidation and exudation...

.

Bacteria

Bacteria are the most common cause of community acquired pneumonia, with Streptococcus pneumoniae
Streptococcus pneumoniae
Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, aerotolerant anaerobic member of the genus Streptococcus. A significant human pathogenic bacterium, S...

isolated in nearly 50% of cases. Other commonly isolated bacteria include: Haemophilus influenzae
Haemophilus influenzae
Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H...

in 20%, Chlamydophila pneumoniae
Chlamydophila pneumoniae
Chlamydophila pneumoniae is a species of Chlamydophila, an obligate intracellular bacteria that infects humans and is a major cause of pneumonia....

in 13%, Mycoplasma pneumoniae
Mycoplasma pneumoniae
Mycoplasma pneumoniae is a very small bacterium in the class Mollicutes.It causes the disease mycoplasma pneumonia, a form of atypical bacterial pneumonia, and is related to cold agglutinin disease.-Cell wall/Treatment:...

 in 3%,, Staphylococcus aureus
Staphylococcus aureus
Staphylococcus aureus is a facultative anaerobic Gram-positive coccal bacterium. It is frequently found as part of the normal skin flora on the skin and nasal passages. It is estimated that 20% of the human population are long-term carriers of S. aureus. S. aureus is the most common species of...

, Moraxella catarrhalis
Moraxella catarrhalis
Moraxella catarrhalis is a fastidious, nonmotile, Gram-negative, aerobic, oxidase-positive diplococcus that can cause infections of the respiratory system, middle ear, eye, central nervous system and joints of humans.-History:...

, Legionella pneumophila
Legionella pneumophila
Legionella pneumophila is a thin, ærobic, pleomorphic, flagellated, non-spore forming, Gram-negative bacterium of the genus Legionella. L. pneumophila is the primary human pathogenic bacterium in this group and is the causative agent of legionellosis or Legionnaires' disease.-Characterization:L...

and gram-negative bacilli.

Risk factors for infection depend on the organism involved. Alcoholism
Alcoholism
Alcoholism is a broad term for problems with alcohol, and is generally used to mean compulsive and uncontrolled consumption of alcoholic beverages, usually to the detriment of the drinker's health, personal relationships, and social standing...

 is associated with Streptococcus pneumoniae, anaerobic organism
Anaerobic organism
An anaerobic organism or anaerobe is any organism that does not require oxygen for growth. It could possibly react negatively and may even die if oxygen is present...

s, and Mycobacterium tuberculosis, smoking is associated with Streptococcus pneumoniae, Haemophilus influenzae, Moraxella catarrhalis, and Legionella pneumophila, exposure to bird with Chlamydia psittaci, farm animals with Coxiella burnetti, aspiration of stomach contents with anaerobes, and cystic fibrosis
Cystic fibrosis
Cystic fibrosis is a recessive genetic disease affecting most critically the lungs, and also the pancreas, liver, and intestine...

 with Pseudomonas aeruginosa
Pseudomonas aeruginosa
Pseudomonas aeruginosa is a common bacterium that can cause disease in animals, including humans. It is found in soil, water, skin flora, and most man-made environments throughout the world. It thrives not only in normal atmospheres, but also in hypoxic atmospheres, and has, thus, colonized many...

and Staphylococcus aureus. Streptococcus pneumoniae is more common in the winter.

Viruses

In adults virus
Virus
A virus is a small infectious agent that can replicate only inside the living cells of organisms. Viruses infect all types of organisms, from animals and plants to bacteria and archaea...

es account for approximately a third of pneumonia cases. Commonly implicated agents include: rhinovirus
Rhinovirus
Human rhinoviruses are the most common viral infective agents in humans and are the predominant cause of the common cold. Rhinovirus infection proliferates in temperatures between 33–35 °C , and this may be why it occurs primarily in the nose...

es,coronavirus
Coronavirus
Coronaviruses are species in the genera of virus belonging to the subfamily Coronavirinae in the family Coronaviridae. Coronaviruses are enveloped viruses with a positive-sense single-stranded RNA genome and a helical symmetry. The genomic size of coronaviruses ranges from approximately 16 to 31...

es, influenza virus, respiratory syncytial virus (RSV), adenovirus, and parainfluenza. Herpes simplex virus
Herpes simplex virus
Herpes simplex virus 1 and 2 , also known as Human herpes virus 1 and 2 , are two members of the herpes virus family, Herpesviridae, that infect humans. Both HSV-1 and HSV-2 are ubiquitous and contagious...

 is a rare cause of pneumonia, except in newborns. People with weakened immune systems are at increased risk of pneumonia caused by cytomegalovirus
Cytomegalovirus
Cytomegalovirus is a viral genus of the viral group known as Herpesviridae or herpesviruses. It is typically abbreviated as CMV: The species that infects humans is commonly known as human CMV or human herpesvirus-5 , and is the most studied of all cytomegaloviruses...

 (CMV).

Fungi

Fungal pneumonia is uncommon, but it may occur in individuals with weakened immune systems
Immunodeficiency
Immunodeficiency is a state in which the immune system's ability to fight infectious disease is compromised or entirely absent. Immunodeficiency may also decrease cancer immunosurveillance. Most cases of immunodeficiency are acquired but some people are born with defects in their immune system,...

 due to AIDS
AIDS
Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...

, immunosuppressive drug
Immunosuppressive drug
Immunosuppressive drugs or immunosuppressive agents are drugs that inhibit or prevent activity of the immune system. They are used in immunosuppressive therapy to:...

s, or other medical problems. The pathophysiology of pneumonia caused by fungi is similar to that of bacterial pneumonia. Fungal pneumonia is most often caused by Histoplasma capsulatum
Histoplasmosis
Histoplasmosis is a disease caused by the fungus Histoplasma capsulatum. Symptoms of this infection vary greatly, but the disease primarily affects the lungs...

, blastomyces, Cryptococcus neoformans
Cryptococcus neoformans
Cryptococcus neoformans is an encapsulated yeast that can live in both plants and animals. Its teleomorph is Filobasidiella neoformans, a filamentous fungus belonging to the class Tremellomycetes. It is often found in pigeon excrement....

, Pneumocystis jiroveci, and Coccidioides immitis
Coccidioides immitis
Coccidioides immitis is a pathogenic fungus that resides in the soil in certain parts of the southwestern United States, northern Mexico, and a few other areas in the Western Hemisphere....

. Histoplasmosis
Histoplasmosis
Histoplasmosis is a disease caused by the fungus Histoplasma capsulatum. Symptoms of this infection vary greatly, but the disease primarily affects the lungs...

 is most common in the Mississippi River basin
Mississippi embayment
The Mississippi Embayment is a physiographic feature in the south-central United States, part of the Mississippi Alluvial Plain. It is essentially a northward continuation of the fluvial sediments of the Mississippi River Delta to its confluence with the Ohio River at Cairo, Illinois. The embayment...

, and coccidioidomycosis
Coccidioidomycosis
Coccidioidomycosis is a fungal disease caused by Coccidioides immitis or C. posadasii. It is endemic in certain parts of Arizona, California, Nevada, New Mexico, Texas, Utah and northwestern Mexico.C...

 is most common in the southwestern United States
Southwestern United States
The Southwestern United States is a region defined in different ways by different sources. Broad definitions include nearly a quarter of the United States, including Arizona, California, Colorado, Nevada, New Mexico, Oklahoma, Texas and Utah...

.

Parasites

A variety of parasites can affect the lungs. These parasites typically enter the body through the skin or the mouth. Once inside the body, they travel to the lungs, usually through the blood. In parasitic pneumonia, as with other kinds of pneumonia, a combination of cellular destruction and immune response causes disruption of oxygen transportation. One type of white blood cell, the eosinophil, responds vigorously to parasite infection. Eosinophils in the lungs can lead to eosinophilic pneumonia
Eosinophilic pneumonia
Eosinophilic pneumonia is a disease in which a certain type of white blood cell called an eosinophil accumulates in the lung. These cells cause disruption of the normal air spaces where oxygen is extracted from the atmosphere. Several different kinds of eosinophilic pneumonia exist and can occur...

, thus complicating the underlying parasitic pneumonia. The most common parasites causing pneumonia are Toxoplasma gondii
Toxoplasma gondii
Toxoplasma gondii is a species of parasitic protozoa in the genus Toxoplasma. The definitive host of T. gondii is the cat, but the parasite can be carried by many warm-blooded animals . Toxoplasmosis, the disease of which T...

, Strongyloides stercoralis
Strongyloides stercoralis
Strongyloides stercoralis, also known as the threadworm, is the scientific name of a human parasitic roundworm causing the disease of strongyloidiasis....

, and Ascariasis
Ascariasis
Ascariasis is a human disease caused by the parasitic roundworm Ascaris lumbricoides. Perhaps as many as one quarter of the world's people are infected, with rates of 45% in Latin America and 95% in parts of Africa. Ascariasis is particularly prevalent in tropical regions and in areas of poor...

.

Idiopathic

Idiopathic interstitial pneumonia or noninfectious pneumonia are a class of diffuse lung diseases. They include: diffuse alveolar damage
Diffuse alveolar damage
Diffuse alveolar damage is a histological pattern in lung disease. It is seen in acute respiratory distress syndrome , transfusion related acute lung injury and acute interstitial pneumonia .-Prevalence:...

, organizing pneumonia, nonspecific interstitial pneumonia
Nonspecific interstitial pneumonia
Non-specific interstitial pneumonia is a form of idiopathic interstitial pneumonia.It has a better prognosis than usual interstitial pneumonia....

, lymphocytic interstitial pneumonia
Lymphocytic interstitial pneumonia
Lymphocytic interstitial pneumonia is a syndrome secondary to autoimmune and other lymphoproliferative disorders. Symptoms include fever, cough, and shortness of breath...

, desquamative interstitial pneumonia
Desquamative interstitial pneumonia
Desquamative interstitial pneumonia is a form of idiopathic interstitial pneumonia featuring elevated levels of macrophages.Its name is derived from the former belief that these macrophages were pneumocytes that had desquamated....

, respiratory bronchiolitis interstitial lung disease, and usual interstitial pneumonia
Usual interstitial pneumonia
Usual interstitial pneumonia is a form of lung disease characterized by progressive scarring of both lungs. The scarring involves the supporting framework of the lung. UIP is thus classified as a form of interstitial lung disease. The term "usual" refers to the fact that UIP is the most common...

.

Pathophysiology

Pneumonia frequently starts as a upper respiratory tract infection
Upper respiratory tract infection
Upper respiratory tract infections are the illnesses caused by an acute infection which involves the upper respiratory tract: nose, sinuses, pharynx or larynx...

 that moves into the lower respiratory tract.

Viral

Viruses invade cells in order to reproduce. Typically, a virus reaches the lungs when airborne droplets are inhaled through the mouth
Mouth
The mouth is the first portion of the alimentary canal that receives food andsaliva. The oral mucosa is the mucous membrane epithelium lining the inside of the mouth....

 or nose
Human nose
The visible part of the human nose is the protruding part of the face that bears the nostrils. The shape of the nose is determined by the ethmoid bone and the nasal septum, which consists mostly of cartilage and which separates the nostrils...

. Once in the lungs, the virus invades the cells lining the airways and alveoli. This invasion often leads to cell death, either from damage to the cell by the virus, or from a protective process called apoptosis
Apoptosis
Apoptosis is the process of programmed cell death that may occur in multicellular organisms. Biochemical events lead to characteristic cell changes and death. These changes include blebbing, cell shrinkage, nuclear fragmentation, chromatin condensation, and chromosomal DNA fragmentation...

 in which the infected cell destroys itself before it can be used as a conduit for virus reproduction. When the immune system responds to the viral infection, even more lung damage occurs. White blood cell
White blood cell
White blood cells, or leukocytes , are cells of the immune system involved in defending the body against both infectious disease and foreign materials. Five different and diverse types of leukocytes exist, but they are all produced and derived from a multipotent cell in the bone marrow known as a...

s, mainly lymphocyte
Lymphocyte
A lymphocyte is a type of white blood cell in the vertebrate immune system.Under the microscope, lymphocytes can be divided into large lymphocytes and small lymphocytes. Large granular lymphocytes include natural killer cells...

s, activate certain chemical cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...

s which allow fluid to leak into the alveoli. This combination of cell destruction and fluid-filled alveoli interrupts the normal transportation of oxygen into the bloodstream.

As well as damaging the lungs, many viruses affect other organ
Organ (anatomy)
In biology, an organ is a collection of tissues joined in structural unit to serve a common function. Usually there is a main tissue and sporadic tissues . The main tissue is the one that is unique for the specific organ. For example, main tissue in the heart is the myocardium, while sporadic are...

s and thus disrupt many body functions. Viruses can also make the body more susceptible to other bacterial infections; in this way bacterial pneumonia can arise as a co-morbid condition.

Bacterial

Bacteria typically enter the lung when airborne droplets are inhaled, but can also reach the lung through the bloodstream when there is an infection in another part of the body. Many bacteria live in parts of the upper respiratory tract
Upper respiratory tract
The upper respiratory tract or upper airway primarily refers to the parts of the respiratory system lying outside of the thorax or above the sternal angle. Another definition commomly used in medicine is the airway above the glottis or vocal cords...

, such as the nose, mouth and sinuses, and can easily be inhaled into the alveoli. Once inside, bacteria may invade the spaces between cells and between alveoli through connecting pores. This invasion triggers the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

 to send neutrophils, a type of defensive white blood cell, to the lungs. The neutrophils engulf and kill the offending organisms, and also release cytokine
Cytokine
Cytokines are small cell-signaling protein molecules that are secreted by the glial cells of the nervous system and by numerous cells of the immune system and are a category of signaling molecules used extensively in intercellular communication...

s, causing a general activation of the immune system. This leads to the fever, chills, and fatigue common in bacterial and fungal pneumonia. The neutrophils, bacteria, and fluid from surrounding blood vessels fill the alveoli and interrupt normal oxygen transportation.

Diagnosis

Pneumonia is typically diagnosed based on a combination of physical signs and a chest X-ray
Chest X-ray
In medicine, a chest radiograph, commonly called a chest X-ray , is a projection radiograph of the chest used to diagnose conditions affecting the chest, its contents, and nearby structures...

. Confirming the underlying cause can be difficult, however, with no definitive test able to distinguish between bacterial and not-bacterial origin. The World Health Organization has defined pneumonia in children clinically based on a either a cough
Cough
A cough is a sudden and often repetitively occurring reflex which helps to clear the large breathing passages from secretions, irritants, foreign particles and microbes...

 or difficulty breathing and a rapid respiratory rate, chest indrawing, or a decreased level of consciousness. A rapid respiratory rate is defined as greater than 60 breaths per minute in children under 2 months old, 50 breaths per minute in children two months to one year old, or greater than 40 breaths per minute in children one to five years old. In children, an increased respiratory rate and lower chest indrawing are more sensitive
Sensitivity and specificity
Sensitivity and specificity are statistical measures of the performance of a binary classification test, also known in statistics as classification function. Sensitivity measures the proportion of actual positives which are correctly identified as such Sensitivity and specificity are statistical...

 than hearing chest crackles with a stethoscope
Stethoscope
The stethoscope is an acoustic medical device for auscultation, or listening to the internal sounds of an animal body. It is often used to listen to lung and heart sounds. It is also used to listen to intestines and blood flow in arteries and veins...

.

In adults investigations are generally not needed in mild cases as if all vital signs and auscultation are normal the risk of pneumonia is very low.In those requiring admission to a hospital, pulse oximetry
Pulse oximetry
Pulse oximetry is a non-invasive method allowing the monitoring of the oxygenation of a patient's hemoglobin.A sensor is placed on a thin part of the patient's body, usually a fingertip or earlobe, or in the case of an infant, across a foot....

, chest radiography, and blood test
Blood test
A blood test is a laboratory analysis performed on a blood sample that is usually extracted from a vein in the arm using a needle, or via fingerprick....

s including a complete blood count
Complete blood count
A complete blood count , also known as full blood count or full blood exam or blood panel, is a test panel requested by a doctor or other medical professional that gives information about the cells in a patient's blood...

, serum electrolytes, C-reactive protein
C-reactive protein
C-reactive protein is a protein found in the blood, the levels of which rise in response to inflammation...

, and possibly liver function tests
Liver function tests
Liver function tests , are groups of clinical biochemistry laboratory blood assays designed to give information about the state of a patient's liver. The parameters measured include PT/INR, aPTT, albumin, billirubin and others...

 are recommended. The diagnosis of influenza-like illness
Influenza-like illness
Influenza-like illness , also known as acute respiratory infection and flu-like syndrome, is a medical diagnosis of possible influenza or other illness causing a set of common symptoms, with SARI referring to Severe Acute Respiratory Infection.Symptoms commonly include fever, shivering, chills,...

 can be made based on presenting signs and symptoms however verification of an influenza infection requires testing. Thus treatment is frequently based on the presence of influenza in the community or a rapid influenza test
Rapid influenza diagnostic test
A rapid influenza diagnostic test is a type of antigen detection test that detect influenza viral nucleoprotein antigen. Commercially available RIDTs can provide results within 30 minutes or less...

.

Imaging

A chest radiograph is frequently used in diagnosis. In people with mild disease imaging is only needed in those with potential complications, those who have not improved with treatment, or those in which the cause in uncertain. If a person is sufficiently sick to require hospitalization, a chest radiograph is recommended. Findings do not always correlate with severity of disease and do not reliably distinguish between bacterial versus viral infection.

X-ray signs of bacterial community acquired pneumonia classically show lung consolidation of one lung segmental lobe
Bronchopulmonary segment
Each of the tertiary bronchi serves a specific bronchopulmonary segment. These segments each have their own artery. Thus, each bronchopulmonary segment is supplied by a bronchus, and two arteries, a pulmonary artery and a bronchial artery which run together through the center of the segment...

. However, radiographic findings may be variable, especially in other types of pneumonia. Aspiration pneumonia may present with bilateral opacities primarily in the bases of the lungs and on the right side. Radiographs of viral pneumonia cases may appear normal, hyper-inflated, have bilateral patchy areas, or present similar to bacterial pneumonia with lobar consolidation. A CT scan can give additional information in indeterminate cases.

Microbiology

For people managed in the community figuring out the causative agent is not cost effective, and typically does not alter management. For those who do not respond to treatment, sputum culture
Sputum culture
A sputum culture is a test to detect and identify bacteria or fungi that infect the lungs or breathing passages. Sputum is a thick fluid produced in the lungs and in the adjacent airways. A sample of sputum is placed in a sterile container and sent to the laboratory for testing...

 should be considered, and culture for Mycobacterium tuberculosis
Mycobacterium tuberculosis
Mycobacterium tuberculosis is a pathogenic bacterial species in the genus Mycobacterium and the causative agent of most cases of tuberculosis . First discovered in 1882 by Robert Koch, M...

should be carried out in those with a chronic productive cough. Testing for other specific organisms may be recommended during outbreaks, for public health reasons. In those who are hospitalized for severe disease both sputum and blood cultures are recommended. Viral infections can be confirmed via detection of either the virus or its antigen
Antigen
An antigen is a foreign molecule that, when introduced into the body, triggers the production of an antibody by the immune system. The immune system will then kill or neutralize the antigen that is recognized as a foreign and potentially harmful invader. These invaders can be molecules such as...

s with culture
Culture
Culture is a term that has many different inter-related meanings. For example, in 1952, Alfred Kroeber and Clyde Kluckhohn compiled a list of 164 definitions of "culture" in Culture: A Critical Review of Concepts and Definitions...

 or polymerase chain reaction
Polymerase chain reaction
The polymerase chain reaction is a scientific technique in molecular biology to amplify a single or a few copies of a piece of DNA across several orders of magnitude, generating thousands to millions of copies of a particular DNA sequence....

 (PCR) among other techniques. With routine microbiological testing a causative agent is determined in only 15% of cases.

Differential diagnosis

Several diseases can present similar to pneumonia, including: chronic obstructive pulmonary disease
Chronic obstructive pulmonary disease
Chronic obstructive pulmonary disease , also known as chronic obstructive lung disease , chronic obstructive airway disease , chronic airflow limitation and chronic obstructive respiratory disease , is the co-occurrence of chronic bronchitis and emphysema, a pair of commonly co-existing diseases...

 (COPD), asthma
Asthma
Asthma is the common chronic inflammatory disease of the airways characterized by variable and recurring symptoms, reversible airflow obstruction, and bronchospasm. Symptoms include wheezing, coughing, chest tightness, and shortness of breath...

, pulmonary edema
Pulmonary edema
Pulmonary edema , or oedema , is fluid accumulation in the air spaces and parenchyma of the lungs. It leads to impaired gas exchange and may cause respiratory failure...

, bronchiectasis
Bronchiectasis
Bronchiectasis is a disease state defined by localized, irreversible dilation of part of the bronchial tree caused by destruction of the muscle and elastic tissue. It is classified as an obstructive lung disease, along with emphysema, bronchitis, asthma, and cystic fibrosis...

, lung cancer
Lung cancer
Lung cancer is a disease characterized by uncontrolled cell growth in tissues of the lung. If left untreated, this growth can spread beyond the lung in a process called metastasis into nearby tissue and, eventually, into other parts of the body. Most cancers that start in lung, known as primary...

 and pulmonary emboli. Unlike pneumonia, asthma and COPD typically present with wheezing, pulmonary edema presents with an abnormal electrocardiogram
Electrocardiogram
Electrocardiography is a transthoracic interpretation of the electrical activity of the heart over a period of time, as detected by electrodes attached to the outer surface of the skin and recorded by a device external to the body...

, cancer and bronchiectasis present with a cough of longer duration, and pulmonary emboli presents with acute onset sharp chest pain and shortness of breath.

Prevention

Prevention includes vaccination, environmental measures, and appropriately treating other diseases.

Vaccination

Vaccination
Vaccination
Vaccination is the administration of antigenic material to stimulate the immune system of an individual to develop adaptive immunity to a disease. Vaccines can prevent or ameliorate the effects of infection by many pathogens...

 is effective for preventing certain bacterial and viral pneumonias in both children and adults.

Influenza vaccines are modestly effective against influenza A and B. The Center for Disease Control and Prevention (CDC) recommends that everyone 6 months and older get yearly vaccination. When an influenza outbreak is occurring, medications such as amantadine
Amantadine
Amantadine is the organic compound known formally as 1-adamantylamine or 1-aminoadamantane. The molecule consists of adamantane backbone that has an amino group substituted at one of the four methyne positions. This pharmaceutical is sold under the name Symmetrel for use both as an antiviral and an...

, rimantadine
Rimantadine
Rimantadine is an orally administered antiviral drug used to treat, and in rare cases prevent, influenzavirus A infection. When taken within one to two days of developing symptoms, rimantadine can shorten the duration and moderate the severity of influenza. Both rimantadine and the similar drug...

, zanamivir, and oseltamivir
Oseltamivir
Oseltamivir INN , an antiviral drug, slows the spread of influenza virus between cells in the body by stopping the virus from chemically cutting ties with its host cell; median time to symptom alleviation is reduced by 0.5–1 day. The drug is sold under the trade name Tamiflu, and is taken orally...

 can help prevent influenza.

Vaccinations against Haemophilus influenzae
Haemophilus influenzae
Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H...

and Streptococcus pneumoniae
Streptococcus pneumoniae
Streptococcus pneumoniae, or pneumococcus, is Gram-positive, alpha-hemolytic, aerotolerant anaerobic member of the genus Streptococcus. A significant human pathogenic bacterium, S...

have good evidence to support their use. Vaccinating children against Streptococcus pneumoniae has also led to a decreased incidence of these infections in adults, because many adults acquire infections from children. A vaccine against Streptococcus pneumoniae
Pneumococcal polysaccharide vaccine
Pneumococcal polysaccharide vaccine — the latest version is known as Pneumovax 23 — is the first pneumococcal vaccine, the first vaccine derived from a capsular polysaccharide, and an important landmark in medical history...

 is also available for adults, and has been found to decrease the risk of invasive pneumococcal disease.

Other

Appropriately treating underlying illnesses (such as AIDS
AIDS
Acquired immune deficiency syndrome or acquired immunodeficiency syndrome is a disease of the human immune system caused by the human immunodeficiency virus...

) can decrease a person's risk of pneumonia.

There are several ways to prevent pneumonia in newborn infant
Infant
A newborn or baby is the very young offspring of a human or other mammal. A newborn is an infant who is within hours, days, or up to a few weeks from birth. In medical contexts, newborn or neonate refers to an infant in the first 28 days after birth...

s. Testing pregnant women for Group B Streptococcus
Group B Streptococcus
Infection with Group B Streptococcus , also known as 'Streptococcus agalactiae' and more colloquially as Strep B and group B Strep, can cause serious illness and sometimes death, especially in newborn infants, the elderly, and patients with compromised immune systems...

 and Chlamydia trachomatis
Chlamydia trachomatis
Chlamydia trachomatis, an obligate intracellular human pathogen, is one of three bacterial species in the genus Chlamydia. C. trachomatis is a Gram-negative bacteria, therefore its cell wall components retain the counter-stain safranin and appear pink under a light microscope.The inclusion bodies...

, and giving antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

 treatment, if needed, reduces pneumonia in infants. Suctioning the mouth and throat of infants with meconium
Meconium
Meconium is the earliest stools of an infant. Unlike later feces, meconium is composed of materials ingested during the time the infant spends in the uterus: intestinal epithelial cells, lanugo, mucus, amniotic fluid, bile, and water. Meconium is almost sterile, unlike later feces, is viscous and...

-stained amniotic fluid
Amniotic fluid
Amniotic fluid or liquor amnii is the nourishing and protecting liquid contained by the amniotic sac of a pregnant woman.- Development of amniotic fluid :...

 decreases the rate of aspiration pneumonia
Aspiration pneumonia
Aspiration pneumonia is bronchopneumonia that develops due to the entrance of foreign materials into the bronchial tree, usually oral or gastric contents...

.

Management

CURB-65
Symptom Points
Confusion
1
Urea>7mmol/l
1
Respiratory rate>30
1
SBP<90mmHg, DBP<60mmHg
1
Age>=65
1

Typically, oral antibiotics, rest, simple analgesics, and fluids are sufficient for complete resolution. However, those with other medical conditions, the elderly, or those with significant trouble breathing may require more advanced care. If the symptoms worsen, the pneumonia does not improve with home treatment, or complications occur, hospitalization may be required. Worldwide, approximately 7–13% of cases in children result in hospitalization while in the developed world between 22–42% of adults with community acquired pneumonia are admitted. The CURB-65 score is useful for determining the need for admission in adults. If the score is 0 or 1 people can typically be managed at home, if it is 2 a short hospital stay or close follow up is needed, if it is 3–5 hospitalization is recommended. In children those with respiratory distress
Dyspnea
Dyspnea , shortness of breath , or air hunger, is the subjective symptom of breathlessness.It is a normal symptom of heavy exertion but becomes pathological if it occurs in unexpected situations...

 or oxygen saturation's of less than 90% should be hospitalized. The utility of chest physiotherapy
Chest physiotherapy
Chest physiotherapy is a broad, non-specific term used to describe treatments generally performed by physiotherapists and respiratory therapists whereby breathing is improved by the indirect removal of mucus from the breathing passages of a patient...

 in pneumonia has not yet been determined. Over the counter cough medicine
Cough medicine
A cough medicine is a medicinal drug used in an attempt to treat coughing and related conditions. For dry coughs, treatment with cough suppressants may be attempted to suppress the body's urge to cough...

 has not been found to be effective.

Bacterial

Antibiotic
Antibiotic
An antibacterial is a compound or substance that kills or slows down the growth of bacteria.The term is often used synonymously with the term antibiotic; today, however, with increased knowledge of the causative agents of various infectious diseases, antibiotic has come to denote a broader range of...

s improve outcomes in those with bacterial pneumonia. Initially antibiotic choice depends on the characteristics of the person affected, such as age, underlying health, and the location the infection was acquired. In the UK empiric treatment
Empiric therapy
Empiric therapy is a medical term referring to the initiation of treatment prior to determination of a firm diagnosis. It may be thought of as taking the initiative against an anticipated and likely cause of infectious disease. It is most often used when antibiotics are given to a person before the...

 with amoxicillin
Amoxicillin
Amoxicillin , formerly amoxycillin , and abbreviated amox, is a moderate-spectrum, bacteriolytic, β-lactam antibiotic used to treat bacterial infections caused by susceptible microorganisms. It is usually the drug of choice within the class because it is better absorbed, following oral...

 is recommended first line for community-acquired pneumonia
Community-acquired pneumonia
Community-acquired pneumonia is a term used to describe one of several diseases in which individuals who have not recently been hospitalized develop an infection of the lungs . CAP is a common illness and can affect people of all ages. CAP often causes problems like difficulty in breathing, fever,...

 with doxycycline
Doxycycline
Doxycycline INN is a member of the tetracycline antibiotics group, and is commonly used to treat a variety of infections. Doxycycline is a semisynthetic tetracycline invented and clinically developed in the early 1960s by Pfizer Inc. and marketed under the brand name Vibramycin. Vibramycin...

 or clarithromycin
Clarithromycin
Clarithromycin is a macrolide antibiotic used to treat pharyngitis, tonsillitis, acute maxillary sinusitis, acute bacterial exacerbation of chronic bronchitis, pneumonia , skin and skin structure infections...

 as alternatives. In North America
North America
North America is a continent wholly within the Northern Hemisphere and almost wholly within the Western Hemisphere. It is also considered a northern subcontinent of the Americas...

, where the "atypical" forms of community-acquired pneumonia are more common, macrolide
Macrolide
The macrolides are a group of drugs whose activity stems from the presence of a macrolide ring, a large macrocyclic lactone ring to which one or more deoxy sugars, usually cladinose and desosamine, may be attached. The lactone rings are usually 14-, 15-, or 16-membered...

s (such as azithromycin
Azithromycin
Azithromycin is an azalide, a subclass of macrolide antibiotics. Azithromycin is one of the world's best-selling antibiotics...

), and doxycycline have displaced amoxicillin as first-line outpatient treatment in adults. In children with mild or moderate symptoms amoxicillin is still the first line. The use of fluoroquinolones in uncomplicated cases is discouraged due to concerns of side effects and resistance. The duration of treatment has traditionally been seven to ten days, but there is increasing evidence that short courses (three to five days) are similarly effective. Antibiotics recommended for hospital-acquired pneumonia include third- and fourth-generation cephalosporins, carbapenem
Carbapenem
Carbapenems are a class of β-lactam antibiotics with a broad spectrum of antibacterial activity. They have a structure that renders them highly resistant to most β-lactamases...

s, fluoroquinolones, aminoglycoside
Aminoglycoside
An aminoglycoside is a molecule or a portion of a molecule composed of amino-modifiedsugars.Several aminoglycosides function as antibiotics that are effective against certain types of bacteria...

s, and vancomycin
Vancomycin
Vancomycin INN is a glycopeptide antibiotic used in the prophylaxis and treatment of infections caused by Gram-positive bacteria. It has traditionally been reserved as a drug of "last resort", used only after treatment with other antibiotics had failed, although the emergence of...

. These antibiotics are often given intravenously
Intravenous therapy
Intravenous therapy or IV therapy is the infusion of liquid substances directly into a vein. The word intravenous simply means "within a vein". Therapies administered intravenously are often called specialty pharmaceuticals...

, and may be used in combination.

Viral

Neuraminidase inhibitors may be used to treat viral pneumonia
Viral pneumonia
Viral pneumonia is a pneumonia caused by a virus.Viruses are one of the two major causes of pneumonia, the other being bacteria; less common causes are fungi and parasites...

 caused by influenza viruses (influenza A and influenza B). No specific antiviral
Antiviral
Antiviral may refer to:*Antiviral drug*Antiviral protein*Antivirus software*Antiviral Therapy, an academic journal...

 medications are recommended for other types of community acquired viral pneumonias including SARS coronavirus, adenovirus, hantavirus
Hantavirus
Hantaviruses are negative sense RNA viruses in the Bunyaviridae family. Humans may be infected with hantaviruses through rodent bites, urine, saliva or contact with rodent waste products...

, and parainfluenza virus. Influenza A may be treated with rimantadine
Rimantadine
Rimantadine is an orally administered antiviral drug used to treat, and in rare cases prevent, influenzavirus A infection. When taken within one to two days of developing symptoms, rimantadine can shorten the duration and moderate the severity of influenza. Both rimantadine and the similar drug...

 or amantadine
Amantadine
Amantadine is the organic compound known formally as 1-adamantylamine or 1-aminoadamantane. The molecule consists of adamantane backbone that has an amino group substituted at one of the four methyne positions. This pharmaceutical is sold under the name Symmetrel for use both as an antiviral and an...

, while influenza A or B may be treated with oseltamivir
Oseltamivir
Oseltamivir INN , an antiviral drug, slows the spread of influenza virus between cells in the body by stopping the virus from chemically cutting ties with its host cell; median time to symptom alleviation is reduced by 0.5–1 day. The drug is sold under the trade name Tamiflu, and is taken orally...

, zanamivir or peramivir
Peramivir
Peramivir is an experimental antiviral drug developed by BioCryst Pharmaceuticals for the treatment of influenza. It has been authorized for the emergency use of treatment of certain hospitalized patients with known or suspected 2009 H1N1 influenza....

. These are of most benefit if they are started within 48 hours of the onset of symptoms. Many strains of H5N1
H5N1
Influenza A virus subtype H5N1, also known as "bird flu", A or simply H5N1, is a subtype of the influenza A virus which can cause illness in humans and many other animal species...

 influenza A, also known as avian influenza or "bird flu," have shown resistance to rimantadine and amantadine. The use of antibiotics in viral pneumonia is recommended by some experts as it is impossible to rule out a complicating bacterial infection. The British Thoracic Society
British Thoracic Society
The British Thoracic Society was formed in 1982 by the amalgamation of the British Thoracic Association and the Thoracic Society. It is a registered charity and a company limited by guarantee.-Function:...

 recommends that antibiotics be withheld in those with mild disease. The use of corticosteroid
Corticosteroid
Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex. Corticosteroids are involved in a wide range of physiologic systems such as stress response, immune response and regulation of inflammation, carbohydrate metabolism, protein catabolism, blood electrolyte...

s is controversial.

Aspiration

Aspiration pneumonitis
Chemical pneumonitis
Aspiration pneumonitis or chemical pneumonitis is inflammation of the lung caused by aspirating or inhaling irritants. It is sometimes called a "chemical pneumonia", though it is not infectious...

 is generally treated conservatively with antibiotics only indicated for aspiration pneumonia
Aspiration pneumonia
Aspiration pneumonia is bronchopneumonia that develops due to the entrance of foreign materials into the bronchial tree, usually oral or gastric contents...

. The choice of antibiotic will depend on several factors, including the suspected causative organism and whether pneumonia was acquired in the community or developed in a hospital setting. Common options include clindamycin
Clindamycin
Clindamycin rINN is a lincosamide antibiotic. It is usually used to treat infections with anaerobic bacteria but can also be used to treat some protozoal diseases, such as malaria...

, a combination of a beta-lactam antibiotic
Beta-lactam antibiotic
β-Lactam antibiotics are a broad class of antibiotics, consisting of all antibiotic agents that contains a β-lactam nucleus in its molecular structure. This includes penicillin derivatives , cephalosporins , monobactams, and carbapenems...

 and metronidazole
Metronidazole
Metronidazole is a nitroimidazole antibiotic medication used particularly for anaerobic bacteria and protozoa. Metronidazole is an antibiotic, amebicide, and antiprotozoal....

, or an aminoglycoside
Aminoglycoside
An aminoglycoside is a molecule or a portion of a molecule composed of amino-modifiedsugars.Several aminoglycosides function as antibiotics that are effective against certain types of bacteria...

.
Corticosteroid
Corticosteroid
Corticosteroids are a class of steroid hormones that are produced in the adrenal cortex. Corticosteroids are involved in a wide range of physiologic systems such as stress response, immune response and regulation of inflammation, carbohydrate metabolism, protein catabolism, blood electrolyte...

s are commonly used in aspiration pneumonia, but there is no evidence to support their effectiveness.

Prognosis

With treatment, most types of bacterial pneumonia can be cleared within two to four weeks and mortality is very low. Viral pneumonia may last longer, and mycoplasmal pneumonia may take four to six weeks to resolve completely. The eventual outcome of an episode of pneumonia depends on how ill the person is when he or she was first diagnosed. Before the advent of antibiotics mortality was typically 30% for hospitalized patients.

In the United States, about 5% of those diagnosed with pneumococcal pneumonia will die. In cases where the pneumonia progresses to blood infection
Bacteremia
Bacteremia is the presence of bacteria in the blood. The blood is normally a sterile environment, so the detection of bacteria in the blood is always abnormal....

, just over 20% will die.

The death rate (or mortality
Mortality rate
Mortality rate is a measure of the number of deaths in a population, scaled to the size of that population, per unit time...

) also depends on the underlying cause of the pneumonia. Pneumonia caused by Mycoplasma, for instance, is associated with lower mortality. However, about half of the people who develop methicillin-resistant Staphylococcus aureus (MRSA) pneumonia while on a ventilator will die. In regions of the world without advanced health care systems, pneumonia is even more deadly. Limited access to clinics and hospitals, limited access to x-rays, limited antibiotic choices, and inability to diagnose and treat underlying conditions inevitably lead to higher rates of death from pneumonia. For these reasons, the majority of deaths in children under five due to pneumococcal disease occur in developing countries.

Adenovirus can cause severe necrotizing pneumonia in which all or part of a lung has increased translucency
Swyer-James syndrome
Swyer James syndrome is a rare lung disorder found by Physicians Paul Robert Swyer, William Mathiseon Macleod and Radiologist George James in the 1950's in Canada. At the same time J...

 radiographically, which is called Swyer-James Syndrome
Swyer-James syndrome
Swyer James syndrome is a rare lung disorder found by Physicians Paul Robert Swyer, William Mathiseon Macleod and Radiologist George James in the 1950's in Canada. At the same time J...

. Severe adenovirus pneumonia also may result in bronchiolitis obliterans
Bronchiolitis obliterans
Bronchiolitis obliterans , also called obliterative bronchiolitis and constrictive bronchiolitis , is a rare and life-threatening form of non-reversible obstructive lung disease in which the bronchioles are compressed and narrowed by fibrosis and/or inflammation...

, a subacute inflammatory process in which the small airways are replaced by scar tissue
Fibrosis
Fibrosis is the formation of excess fibrous connective tissue in an organ or tissue in a reparative or reactive process. This is as opposed to formation of fibrous tissue as a normal constituent of an organ or tissue...

, resulting in a reduction in lung volume and lung compliance. Sometimes pneumonia can lead to additional complications
Complication (medicine)
Complication, in medicine, is an unfavorable evolution of a disease, a health condition or a medical treatment. The disease can become worse in its severity or show a higher number of signs, symptoms or new pathological changes, become widespread throughout the body or affect other organ systems. A...

. Complications are more frequently associated with bacterial pneumonia than with viral pneumonia. The most important complications include respiratory and circulatory failure and pleural effusions, empyema or abscesses.

Clinical prediction rules

Clinical prediction rules have been developed to more objectively prognosticate outcomes in pneumonia. Although these rules are often used in deciding whether or not to hospitalize the person, they were derived simply to inform on prognosis; neither index was designed or tested as guide to determine whether the person would benefit by hospital admission.
  • Pneumonia severity index
    Pneumonia severity index
    The pneumonia severity index [PSI] or PORT Score is a clinical prediction rule that medical practitioners can use to calculate the probability of morbidity and mortality among patients with community acquired pneumonia....

     (or PORT Score) – online calculator
  • CURB-65 score, which takes into account the severity of symptoms, any underlying diseases, and age – online calculator

Pleural effusion, empyema, and abscess

In pneumonia, a collection of fluid (pleural effusion
Pleural effusion
Pleural effusion is excess fluid that accumulates between the two pleural layers, the fluid-filled space that surrounds the lungs. Excessive amounts of such fluid can impair breathing by limiting the expansion of the lungs during ventilation.-Pathophysiology:...

) often forms in the space that surrounds the lung (the pleural cavity
Pleural cavity
In human anatomy, the pleural cavity is the potential space between the two pleura of the lungs. The pleura is a serous membrane which folds back onto itself to form a two-layered, membrane structure. The thin space between the two pleural layers is known as the pleural cavity; it normally...

). Occasionally, microorganisms will infect this fluid, causing what is called an empyema. To distinguish an empyema from the more common simple parapneumonic effusion
Parapneumonic effusion
A parapneumonic effusion is a type of pleural effusion that arises as a result of a pneumonia, lung abscess, or bronchiectasis. There are three types of parapneumonic effusions: uncomplicated effusions, complicated effusions, and empyema. Uncomplicated effusions generally respond well to...

, the fluid is collected with a needle (thoracentesis
Thoracentesis
Thoracentesis , also known as thoracocentesis or pleural tap, is an invasive procedure to remove fluid or air from the pleural space for diagnostic or therapeutic purposes. A cannula, or hollow needle, is carefully introduced into the thorax, generally after administration of local anesthesia...

), and examined. If this shows evidence of empyema, complete drainage of the fluid may be necessary, often requiring a chest tube
Chest tube
A chest tube is a flexible plastic tube that is inserted through the side of the chest into the pleural space. It is used to remove air or fluid , or pus from the intrathoracic space...

. In severe cases of empyema, surgery
Decortication
Decortication is a medical procedure involving the surgical removal of the surface layer, membrane, or fibrous cover of an organ. The procedure is usually performed when the lung is covered by a thick, inelastic pleural peel restricting lung expansion. In a non-medical aspect, decortication is...

 may be needed. If the infected fluid is not drained, the infection may persist, because antibiotics do not penetrate well into the pleural cavity. If the fluid is sterile, it need only be drained if it is causing symptoms or remains unresolved.

Rarely, bacteria in the lung will form a pocket of infected fluid called a lung abscess
Lung abscess
Lung abscess is necrosis of the pulmonary tissue and formation of cavities containing necrotic debris or fluid caused by microbial infection....

. Lung abscesses can usually be seen with a chest x-ray or chest CT scan. Abscesses typically occur in aspiration pneumonia
Aspiration pneumonia
Aspiration pneumonia is bronchopneumonia that develops due to the entrance of foreign materials into the bronchial tree, usually oral or gastric contents...

, and often contain several types of bacteria. Antibiotics are usually adequate to treat a lung abscess, but sometimes the abscess must be drained by a surgeon
Surgery
Surgery is an ancient medical specialty that uses operative manual and instrumental techniques on a patient to investigate and/or treat a pathological condition such as disease or injury, or to help improve bodily function or appearance.An act of performing surgery may be called a surgical...

 or radiologist
Interventional radiology
Interventional radiology is a specialty of radiology, in which image-guided procedures are used to diagnose and treat a multitude of diseases across all body systems...

.

Respiratory and circulatory failure

Because pneumonia affects the lungs, people with pneumonia often have difficulty breathing, sometimes to the point where mechanical assistance is required. Non-invasive breathing assistance may be helpful, such as with a bi-level positive airway pressure machine. In other cases, placement of an endotracheal tube (breathing tube) may be necessary, and a ventilator
Medical ventilator
A medical ventilator can be defined as any machine designed to mechanically move breatheable air into and out of the lungs, to provide the mechanism of breathing for a patient who is physically unable to breathe, or breathing insufficiently....

 may be used to help the person breathe.

Pneumonia can also cause respiratory failure by triggering acute respiratory distress syndrome
Acute respiratory distress syndrome
Acute respiratory distress syndrome , also known as respiratory distress syndrome or adult respiratory distress syndrome is a serious reaction to various forms of injuries to the lung....

 (ARDS), which results from a combination of infection and inflammatory response. The lungs quickly fill with fluid and become very stiff. This stiffness, combined with severe difficulties extracting oxygen due to the alveolar fluid, creates a need for mechanical ventilation
Mechanical ventilation
In medicine, mechanical ventilation is a method to mechanically assist or replace spontaneous breathing. This may involve a machine called a ventilator or the breathing may be assisted by a physician, respiratory therapist or other suitable person compressing a bag or set of bellows...

.

Sepsis
Sepsis
Sepsis is a potentially deadly medical condition that is characterized by a whole-body inflammatory state and the presence of a known or suspected infection. The body may develop this inflammatory response by the immune system to microbes in the blood, urine, lungs, skin, or other tissues...

 and septic shock
Septic shock
Septic shock is a medical emergency caused by decreased tissue perfusion and oxygen delivery as a result of severe infection and sepsis, though the microbe may be systemic or localized to a particular site. It can cause multiple organ dysfunction syndrome and death...

 are potential complications of pneumonia. Sepsis occurs when microorganisms enter the bloodstream and the immune system
Immune system
An immune system is a system of biological structures and processes within an organism that protects against disease by identifying and killing pathogens and tumor cells. It detects a wide variety of agents, from viruses to parasitic worms, and needs to distinguish them from the organism's own...

 responds by secreting cytokines. Sepsis most often occurs with bacterial pneumonia
Bacterial pneumonia
Bacterial pneumonia is a type of pneumonia caused by bacterial infection.-Sign and symptoms:*Fever*Rigors*Cough*Dyspnea*Chest pain*Pneumococcal pneumonia can cause Hemoptysis-Gram positive:...

; Streptococcus pneumoniae is the most common cause. Individuals with sepsis or septic shock need hospitalization in an intensive care unit
Intensive Care Unit
thumb|220px|ICU roomAn intensive-care unit , critical-care unit , intensive-therapy unit/intensive-treatment unit is a specialized department in a hospital that provides intensive-care medicine...

. They often require intravenous fluids and medications to help keep their blood pressure up. Sepsis can cause liver, kidney, and heart damage, among other problems, and it is often fatal.

Epidemiology

Pneumonia is a common illness affecting approximately 450 million people a year and occurring in all parts of the world. It is a major cause of death among all age groups resulting in 4 million deaths (7% of the world's yearly total). Rates are greatest in children less than five, and adults older than 75 years of age. It occurs about five times more frequently in the developing world versus the developed world. Viral pneumonia accounts for about 200 million cases.

Children

In 2008 pneumonia occurred in approximately 156 million children (151 million in the developing world and 5 million in the developed world). It resulted in 1.6 million deaths, or 28–34% of all deaths in those under five years of age, of which 95% occurred in the developing world. Countries with the greatest burden of disease include: India (43 million), China (21 million) and Pakistan (10 million). It is the leading cause of death among children in low income countries. Many of these deaths occur in the newborn period. The World Health Organization
World Health Organization
The World Health Organization is a specialized agency of the United Nations that acts as a coordinating authority on international public health. Established on 7 April 1948, with headquarters in Geneva, Switzerland, the agency inherited the mandate and resources of its predecessor, the Health...

 estimates that one in three newborn infant deaths are due to pneumonia. Approximately half of these deaths are theoretically preventable, as they are caused by the bacteria for which an effective vaccine is available.

History

Pneumonia has been a common disease throughout human history. The symptoms were described by Hippocrates
Hippocrates
Hippocrates of Cos or Hippokrates of Kos was an ancient Greek physician of the Age of Pericles , and is considered one of the most outstanding figures in the history of medicine...

 (c. 460 BC – 370 BC): "Peripneumonia, and pleuritic affections, are to be thus observed: If the fever be acute, and if there be pains on either side, or in both, and if expiration be if cough be present, and the sputa expectorated be of a blond or livid color, or likewise thin, frothy, and florid, or having any other character different from the common... When pneumonia is at its height, the case is beyond remedy if he is not purged, and it is bad if he has dyspnoea, and urine that is thin and acrid, and if sweats come out about the neck and head, for such sweats are bad, as proceeding from the suffocation, rales, and the violence of the disease which is obtaining the upper hand." However, Hippocrates referred to pneumonia as a disease "named by the ancients." He also reported the results of surgical drainage of empyemas. Maimonides
Maimonides
Moses ben-Maimon, called Maimonides and also known as Mūsā ibn Maymūn in Arabic, or Rambam , was a preeminent medieval Jewish philosopher and one of the greatest Torah scholars and physicians of the Middle Ages...

 (1135–1204 AD) observed "The basic symptoms which occur in pneumonia and which are never lacking are as follows: acute fever, sticking [pleuritic] pain in the side, short rapid breaths, serrated pulse
Pulse
In medicine, one's pulse represents the tactile arterial palpation of the heartbeat by trained fingertips. The pulse may be palpated in any place that allows an artery to be compressed against a bone, such as at the neck , at the wrist , behind the knee , on the inside of the elbow , and near the...

 and cough." This clinical description is quite similar to those found in modern textbooks, and it reflected the extent of medical knowledge through the Middle Ages
Middle Ages
The Middle Ages is a periodization of European history from the 5th century to the 15th century. The Middle Ages follows the fall of the Western Roman Empire in 476 and precedes the Early Modern Era. It is the middle period of a three-period division of Western history: Classic, Medieval and Modern...

 into the 19th century.

Bacteria were first seen in the airways of individuals who died from pneumonia by Edwin Klebs
Edwin Klebs
Theodor Albrecht Edwin Klebs was a German-Swiss pathologist. He is mainly known for his work on infectious diseases. He is the father of Arnold Klebs.-Life:...

 in 1875. Initial work identifying the two common bacterial causes Streptococcus pneumoniae and Klebsiella pneumoniae was performed by Carl Friedländer
Carl Friedländer
Carl Friedländer was a German pathologist and microbiologist who helped discover the bacterial cause of pneumonia in 1882...

 and Albert Fränkel in 1882 and 1884, respectively. Friedländer's initial work introduced the Gram stain
Gram staining
Gram staining is a method of differentiating bacterial species into two large groups ....

, a fundamental laboratory test still used today to identify and categorize bacteria. Christian Gram's paper describing the procedure in 1884 helped differentiate the two different bacteria, and showed that pneumonia could be caused by more than one microorganism.

Sir William Osler
William Osler
Sir William Osler, 1st Baronet was a physician. He was one of the "Big Four" founding professors at Johns Hopkins Hospital as the first Professor of Medicine and founder of the Medical Service there. Sir William Osler, 1st Baronet (July 12, 1849 – December 29, 1919) was a physician. He was...

, known as "the father of modern medicine," appreciated the death and disability cause by pneumonia, describing it as the "captain of the men of death" in 1918, as it had overtaken tuberculosis
Tuberculosis
Tuberculosis, MTB, or TB is a common, and in many cases lethal, infectious disease caused by various strains of mycobacteria, usually Mycobacterium tuberculosis. Tuberculosis usually attacks the lungs but can also affect other parts of the body...

 as one of the leading causes of death in this time. This phrase was originally coined by John Bunyan
John Bunyan
John Bunyan was an English Christian writer and preacher, famous for writing The Pilgrim's Progress. Though he was a Reformed Baptist, in the Church of England he is remembered with a Lesser Festival on 30 August, and on the liturgical calendar of the Episcopal Church on 29 August.-Life:In 1628,...

 in reference to "consumption" (tuberculosis). Osler also described pneumonia as "the old man's friend" as death was often quick and painless when there was many slower more painful ways to die.

Several developments in the 1900s improved the outcome for those with pneumonia. With the advent of penicillin
Penicillin
Penicillin is a group of antibiotics derived from Penicillium fungi. They include penicillin G, procaine penicillin, benzathine penicillin, and penicillin V....

 and other antibiotics, modern surgical techniques, and intensive care in the twentieth century, mortality from pneumonia, which had approached 30%, dropped precipitously in the developed world. Vaccination of infants against Haemophilus influenzae
Haemophilus influenzae
Haemophilus influenzae, formerly called Pfeiffer's bacillus or Bacillus influenzae, Gram-negative, rod-shaped bacterium first described in 1892 by Richard Pfeiffer during an influenza pandemic. A member of the Pasteurellaceae family, it is generally aerobic, but can grow as a facultative anaerobe. H...

type B began in 1988 and led to a dramatic decline in cases shortly thereafter. Vaccination against Streptococcus pneumoniae in adults began in 1977, and in children in 2000, resulting in a similar decline.

Society and culture

Because of the combination of a very high burden of disease in developing countries and a relatively low awareness of the disease in industrialized countries, the global health community has declared November 12 to be World Pneumonia Day
World Pneumonia Day
World Pneumonia Day provides an annual forum for the world to stand together and demand action in the fight against pneumonia. More than 100 organizations representing the interests of children joined forces as the Global Coalition against Child Pneumonia to hold the first World Pneumonia Day on...

, a day for concerned citizens and policy makers to take action against the disease.
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