Host species

In biology, a host is an organism that harbors a parasitic, a mutualistic, or a commensalist guest (symbiont), typically providing nourishment and shelter. Examples include animals playing host to parasitic worms (e.g. nematodes), cells harbouring pathogenic (disease-causing) viruses, a bean plant hosting mutualistic (helpful) nitrogen-fixing bacteria. More specifically in botany, a host plant supplies food resources and acts as a substrate for commensalist insects or other fauna. The host range is the collection of hosts that a parasite can use as a partner.

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

A definitive or primary host is the one in which a parasite reaches maturity and, if possible, reproduces sexually. An intermediate host is one in which the parasite does not do this.

A reservoir host can harbour a pathogen indefinitely with no ill effects. A single reservoir host may be reinfected several times.

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 lifecycles, in which specific developmental stages are completed in a sequence of several different hosts.

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