Host organism

In biology, a host is an organism that harbors a parasitic, a mutual, or a commensal symbiont, typically providing nourishment and shelter. Examples include animals playing host to parasitic worms (e.g. nematodes), cells harbouring a parasitic virus, 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.

Guest is the generic term used for parasites, mutualists and commensals.

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 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.

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