In
biology, a
host is an organism that harbors a
parasite (that is, a
virus, a
bacterium, a
protozoa, or a
fungus), 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 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 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.
