Host (biology)
In biology
Biology is a natural science concerned with the study of life and living organisms, including their structure, function, growth, origin, evolution, distribution, and taxonomy. Biology is a vast subject containing many subdivisions, topics, and disciplines...

, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany
Botany, plant science, or plant biology is a branch of biology that involves the scientific study of plant life. Traditionally, botany also included the study of fungi, algae and viruses...

, a host plant is one that supplies food resources and substrate for certain insects or other fauna
Fauna or faunæ is all of the animal life of any particular region or time. The corresponding term for plants is flora.Zoologists and paleontologists use fauna to refer to a typical collection of animals found in a specific time or place, e.g. the "Sonoran Desert fauna" or the "Burgess shale fauna"...

. Examples of such interactions include a cell
Cell (biology)
The cell is the basic structural and functional unit of all known living organisms. It is the smallest unit of life that is classified as a living thing, and is often called the building block of life. The Alberts text discusses how the "cellular building blocks" move to shape developing embryos....

 being host to a virus, a legume
The Fabaceae or Leguminosae, commonly known as the legume, pea, or bean family, is a large and economically important family of flowering plants. The group is the third largest land plant family, behind only the Orchidaceae and Asteraceae, with 730 genera and over 19,400 species...

 plant hosting helpful nitrogen-fixing bacteria
Rhizobia are soil bacteria that fix nitrogen after becoming established inside root nodules of legumes . Rhizobia require a plant host; they cannot independently fix nitrogen...

, and animal
Animals are a major group of multicellular, eukaryotic organisms of the kingdom Animalia or Metazoa. Their body plan eventually becomes fixed as they develop, although some undergo a process of metamorphosis later on in their life. Most animals are motile, meaning they can move spontaneously and...

s as hosts to parasitic worm
The term worm refers to an obsolete taxon used by Carolus Linnaeus and Jean-Baptiste Lamarck for all non-arthropod invertebrate animals, and stems from the Old English word wyrm. Currently it is used to describe many different distantly-related animals that typically have a long cylindrical...

s, e.g. nematode
The nematodes or roundworms are the most diverse phylum of pseudocoelomates, and one of the most diverse of all animals. Nematode species are very difficult to distinguish; over 28,000 have been described, of which over 16,000 are parasitic. It has been estimated that the total number of nematode...



A host cell is a living cell in which a virus reproduces.

A primary host or definitive host is a host in which the parasite reaches maturity and, if applicable, reproduces sexually.

A secondary host or intermediate host is a host that harbors the parasite only for a short transition period, during which (usually) some developmental stage is completed. For trypanosomes
Trypanosoma brucei
Trypanosoma brucei is a parasitic protist species that causes African trypanosomiasis in humans and nagana in animals in Africa. There are 3 sub-species of T. brucei: T. b. brucei, T. b. gambiense and T. b. rhodesiense.These obligate parasites have two hosts - an insect vector and mammalian host...

, the cause of sleeping sickness,strictly, human
Humans are the only living species in the Homo genus...

s are the secondary host, while the tsetse fly
Tsetse fly
Tsetse , sometimes spelled tzetze and also known as tik-tik flies, are large biting flies that inhabit much of mid-continental Africa between the Sahara and the Kalahari deserts. They live by feeding on the blood of vertebrate animals and are the primary biological vectors of trypanosomes, which...

 is the primary host, given that it has been shown that reproduction occurs in the insect. Cestodes (tapeworms) and other parasitic flatworm
The flatworms, known in scientific literature as Platyhelminthes or Plathelminthes are a phylum of relatively simple bilaterian, unsegmented, soft-bodied invertebrate animals...

s have complex life-cycles, in which specific developmental stages are completed in a sequence of several different hosts.

As the life cycles
Biological life cycle
A life cycle is a period involving all different generations of a species succeeding each other through means of reproduction, whether through asexual reproduction or sexual reproduction...

 of many parasites are not well understood, sometimes the "more important" organism is arbitrarily defined as definitive, and this designation may continue even after it is determined to be incorrect. For example, sludge worms are sometimes considered "intermediate hosts" for whirling disease, even though it is known that the parasite causing the disease reproduces sexually inside them

In Trichinella spiralis, the roundworm that causes trichinosis
Trichinosis, also called trichinellosis, or trichiniasis, is a parasitic disease caused by eating raw or undercooked pork or wild game infected with the larvae of a species of roundworm Trichinella spiralis, commonly called the trichina worm. There are eight Trichinella species; five are...

, a host has both reproductive adults in its digestive tract and immature juveniles in its muscle
Muscle is a contractile tissue of animals and is derived from the mesodermal layer of embryonic germ cells. Muscle cells contain contractile filaments that move past each other and change the size of the cell. They are classified as skeletal, cardiac, or smooth muscles. Their function is to...

s, and is therefore considered both an intermediate host and a definitive host.

A paratenic
In parasitology, the term paratenic describes a host which is not needed for the development of the parasite, but nonetheless serves to maintain the life cycle of the parasite. Alaria americana may serve as an example: the so-called mesocercarial stages of this parasite reside in tadpoles, which...

is similar to an intermediate host, only that it is not needed for the parasite's development cycle to progress. There are also reservoir hosts
Natural reservoir
Natural reservoir or nidus, refers to the long-term host of the pathogen of an infectious disease. It is often the case that hosts do not get the disease carried by the pathogen or it is carried as a subclinical infection and so asymptomatic and non-lethal...

. A reservoir can harbor a pathogen indefinitely with no ill effects. A single reservoir host may be reinfected several times. The difference between a paratenic and reservoir host is that the latter is a primary host, whereas paratenic hosts serve as "dumps" for non-mature stages of a parasite in which they can accumulate in high numbers.

A dead-end host is an intermediate host that does generally not allow transmission to the definitive host, thereby preventing the parasite from completing its development. For example, humans are dead-end hosts for Echinococcus
The genus Echinococcus includes six species of cyclophyllid tapeworms to date, of the family Taeniidae. Infection with Echinococcus results in hydatid disease, also known as echinococcosis....

canine tapeworms. As infected humans are not usually eaten by dogs, foxes etc., the immature Echinococcus - although it causes serious disease in the dead-end host - is unable to infect the primary host and mature.

Host of Predilection is the host preferred by a parasite.

Amplifying host is a host in which the level of pathogen can become high enough that a vector such as a mosquito that feeds on it will probably become infectious.

Host range

The host range or host specificity of a parasite is the collection of hosts that an organism can utilize as a partner. In the case of human parasites, the host range influences the 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...

 of the parasitism or disease. For instance, the production of antigenic shift
Antigenic shift
Antigenic shift is the process by which two or more different strains of a virus, or strains of two or more different viruses, combine to form a new subtype having a mixture of the surface antigens of the two or more original strains...

s in Influenza A virus can result from pigs being infected with the virus from several different hosts (such as human and bird). This co-infection provides an opportunity for mixing of the viral genes between existing strains, thereby producing a new viral strain. An influenza 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...

 produced against an existing viral strain
Strain (biology)
In biology, a strain is a low-level taxonomic rank used in three related ways.-Microbiology and virology:A strain is a genetic variant or subtype of a micro-organism . For example, a "flu strain" is a certain biological form of the influenza or "flu" virus...

 might not be effective against this new strain, which then requires a new influenza vaccine to be prepared for the protection of the human population

See also

  • Aposymbiotic
    Aposymbiosis occurs when symbiotic organisms live apart from one another . Studies have shown that the lifecycles of both the host and the symbiote are affected in some way, usually negative, and that for obligate symbiosis the effects can be drastic...

  • Host cell factor C1
    Host cell factor C1
    Host cell factor is a protein that in humans is encoded by the HCFC1 gene.-Interactions:Host cell factor C1 has been shown to interact with HDAC1, Histone deacetylase 2, OGT, SIN3A, SUDS3, ZBTB17, Sp1 transcription factor, MLL, PPP1CA, POU2F1, PDCD2, WDR5, CREB3 and GABPA.-Further reading:...

  • Intermediate host
    Intermediate host
    A secondary host or intermediate host is a host that harbors the parasite only for a short transition period, during which some developmental stage is completed. For trypanosomes, the cause of sleeping sickness, humans are the primary host, while the tsetse fly is the secondary host...

  • PHI-base
    The Pathogen - Host Interaction database contains expertly curated molecular and biological information on genes proven to affect the outcome of pathogen-host interactions...

     (Pathogen-Host Interaction database)
  • Symbiosis
    Symbiosis is close and often long-term interaction between different biological species. In 1877 Bennett used the word symbiosis to describe the mutualistic relationship in lichens...

  • Vector
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