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In biology, a host is an organism that harbors a parasite, or a mutual or commensal symbiont, typically providing nourishment and shelter. In botany, a host plant is one that supplies food resources and substrate for certain insects or other fauna. Examples of such interactions include a cell being host to a virus, a bean plant hosting helpful nitrogen-fixing bacteria, and animals as hosts to parasitic worms, e.g. nematodes.
The word "guest" is also (more rarely) used in biology to refer to the organism that lives with the host.
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 possible, reproduces sexually.
A reservoir host can harbour a pathogen indefinitely with no ill effects. A single reservoir host may be reinfected several times. A reservoir host is the primary host of a pathogen.
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, the cause of sleeping sickness, strictly, humans are the secondary host, while the tsetse fly is the primary host, given that it has been shown that reproduction occurs in the insect. Cestodes (tapeworms) and other parasitic flatworms have complex life-cycles, in which specific developmental stages are completed in a sequence of several different hosts.
As the life cycles 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, a host has both reproductive adults in its digestive tract and immature juveniles in its muscles, and is therefore considered both an intermediate host and a definitive host.
A paratenic host is similar to an intermediate host, only that it is not needed for the parasite's development cycle to progress. A paratenic hosts serve as "dumps" for non-mature stages of a parasite in which they can accumulate in high numbers.
A dead-end host or incidental 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 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.
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 of the parasitism or disease. For instance, the production of antigenic shifts 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 produced against an existing viral strain 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