Host (biology)

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In biology, a host is an organism that harbors a virus, parasite, mutual partner, or commensal partner, 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: A cell can be host to a virus, a legume plant can be host to helpful nitrogen-fixing bacteria, an animal can be host to a parasitic worm, e.g. a nematode.

A primary host or definitive host is a host in which the parasite grows mature; a secondary host or intermediate host is a host that harbors the parasite only for a short transition period. For trypanosomes, the cause of sleeping sickness, humans are the primary host, while the tsetse fly is the secondary host.

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].

There are also reservoir hosts. These are vertebrates that host an infection while it isn't infecting humans and is used by the disease as a source of maintenance. A single reservoir host may be reinfected several times.