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In biology, a host is an organism that harbors a virus or 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 legume plant hosting helpful nitrogen-fixing bacteria, and animals as hosts to parasitic worms, e.g. nematodes.

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, the cause of sleeping sickness, humans are the primary host, while the tsetse fly is the secondary host. Cestodes (tapeworms) and other parasitic flatworms have complex life-cycles, in which specific developmental stages are completed in a sequence of several different hosts.

A paratenic host 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. These are animals that host a human pathogen while it isn't infecting humans, and are used by the disease as a source of maintenance. 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 which they can accumulate in high numbers.

A dead-end host is an intermediate host that does generally not allow transmission to the definite 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.

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^[1].

• Aposymbiotic
• Intermediate host
• Symbiosis
• Vector

1. ^ Centers for Disease Control and Prevention (2004). The Influenza (Flu) Viruses:Transmission of Influenza Viruses from Animals to People. Retrieved 2005-02-26.