In biology, a host is an organism that harbors a parasite, mutual partner, or commensal partner, typically providing nourishment and shelter.

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.

Related topics

• Symbiosis
• Vector

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