Susceptible individual
Encyclopedia
In epidemiology
Epidemiology
Epidemiology is the study of health-event, health-characteristic, or health-determinant patterns in a population. It is the cornerstone method of public health research, and helps inform policy decisions and evidence-based medicine by identifying risk factors for disease and targets for preventive...

 a susceptible individual (sometimes known simply as a susceptible) is a member of a population
Population
A population is all the organisms that both belong to the same group or species and live in the same geographical area. The area that is used to define a sexual population is such that inter-breeding is possible between any pair within the area and more probable than cross-breeding with individuals...

 who is at risk of becoming infected by a disease, or can not take a certain medicine, antibiotic, etc if he or she is exposed to the infectious agent
Infectious disease
Infectious diseases, also known as communicable diseases, contagious diseases or transmissible diseases comprise clinically evident illness resulting from the infection, presence and growth of pathogenic biological agents in an individual host organism...

.

Susceptible individuals

Susceptibles have been exposed to neither the wild strain of the disease nor a 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...

 against it, and thus have not developed immunity
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...

. Those individuals who have antibodies
Antibody
An antibody, also known as an immunoglobulin, is a large Y-shaped protein used by the immune system to identify and neutralize foreign objects such as bacteria and viruses. The antibody recognizes a unique part of the foreign target, termed an antigen...

 against an 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...

 associated with a particular infectious disease will not be susceptible, even if they did not produce the antibody themselves (for example, infants younger than six months who still have maternal antibodies passed through the placenta
Placenta
The placenta is an organ that connects the developing fetus to the uterine wall to allow nutrient uptake, waste elimination, and gas exchange via the mother's blood supply. "True" placentas are a defining characteristic of eutherian or "placental" mammals, but are also found in some snakes and...

 and from the colostrum
Colostrum
Colostrum is a form of milk produced by the mammary glands of mammals in late pregnancy. Most species will generate colostrum just prior to giving birth...

, and adults who have had a recent injection of antibodies). However, these individuals soon return to the susceptible state as the antibodies are broken down.

Some individuals may have a natural resistance to a particular infectious disease. However, except in some special cases
Malaria
Malaria is a mosquito-borne infectious disease of humans and other animals caused by eukaryotic protists of the genus Plasmodium. The disease results from the multiplication of Plasmodium parasites within red blood cells, causing symptoms that typically include fever and headache, in severe cases...

, these individuals make up such a small proportion of the total population that they can be ignored for the purposes of modelling
Mathematical modelling in epidemiology
It is possible to mathematically model the progress of most infectious diseases to discover the likely outcome of an epidemic or to help manage them by vaccination...

 an epidemic
Epidemic
In epidemiology, an epidemic , occurs when new cases of a certain disease, in a given human population, and during a given period, substantially exceed what is expected based on recent experience...

.

Mathematical model of susceptibility

The proportion of the population who are susceptible to a particular disease is denoted S. Due to the problems mentioned above, it is difficult to know this parameter
Parameter
Parameter from Ancient Greek παρά also “para” meaning “beside, subsidiary” and μέτρον also “metron” meaning “measure”, can be interpreted in mathematics, logic, linguistics, environmental science and other disciplines....

 for a given population. However, in a population with a rectangular population distribution
Population pyramid
A population pyramid, also called an age structure diagram, is a graphical illustration that shows the distribution of various age groups in a population , which forms the shape of a pyramid when the population is growing...

 (such as that of a developed country), it may be estimated by:


Where A is the average age at which the disease is contracted and L is the average life expectancy of the population. To understand the rationale behind this relation, think of A as the length of time spent in the susceptible group (assuming an individual is susceptible before contracting the disease and immune afterwards) and L as the total length of time spent in the population. It thus follows that the proportion of time spent as a susceptible is A/L and, in a population with a rectangular distribution, the proportion of an individual's life spent in one group is representative of the proportion of the population in that group.

The advantage of estimating S in this way is that both the average age of infection and average life expectancy will be well documented, and thus the other parameters needed to calculate S will be easily at hand.

The parameter S is important in the mathematical modelling of epidemics
Mathematical modelling in epidemiology
It is possible to mathematically model the progress of most infectious diseases to discover the likely outcome of an epidemic or to help manage them by vaccination...

.

Susceptibility in virology

Viruses are only able to cause disease or pathologies if they meet several criteria:
  1. The virus is able to enter the cell (called a susceptible state)
  2. There is a sufficient number of viruses within the cell
  3. The virus is able to replicate within the cell (called a permissive
    Permissive
    When a cell or host is defined as permissive in virology, it refers to the fact that the virus is able to circumvent host defenses and is able to replicate. Usually this occurs when the virus has modulated one or several of the host cellular intrinsic defenses, and the host immune system...

    state)


Hence susceptibility only refers to the fact that the virus is able to get into the cell, via having the proper receptor(s), and as a result, despite the fact that a host may be susceptible, the virus may still not be able to cause any pathologies within the host. Reasons for this are varied and may include suppression by the host 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...

, or abortive measures taken by intrinsic cell defenses.

See also

  • Mathematical modelling in epidemiology
    Mathematical modelling in epidemiology
    It is possible to mathematically model the progress of most infectious diseases to discover the likely outcome of an epidemic or to help manage them by vaccination...

  • Immunity (medical)
    Immunity (medical)
    Immunity is a biological term that describes a state of having sufficient biological defenses to avoid infection, disease, or other unwanted biological invasion. Immunity involves both specific and non-specific components. The non-specific components act either as barriers or as eliminators of wide...

  • 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...

  • Immunization
    Immunization
    Immunization, or immunisation, is the process by which an individual's immune system becomes fortified against an agent ....

  • Bugchasing and giftgiving
The source of this article is wikipedia, the free encyclopedia.  The text of this article is licensed under the GFDL.
 
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