host (biology) host (biology) in biology and medicine, a host is a larger organism that harbours a smaller organism; whether a parasitic, a mutualistic, or a commensalist guest (symbiont). the guest is typically provided with nourishment and shelter. examples include animals playing host to parasitic worms (e.g. nematodes), cells harbouring pathogenic (disease-causing) viruses, or a bean plant hosting mutualistic (helpful) nitrogen-fixing bacteria. more specifically in botany, a host plant supplies food resources to micropredators, which have an evolutionarily stable relationship with their hosts similar to ectoparasitism. the host range is the collection of hosts that an organism can use as a partner. the black rat is a reservoir host for bubonic plague. the oriental rat fleas that infest the rats are vectors for the disease. symbiosis symbiosis spans a wide variety of possible relationships between organisms, differing in their permanence and their effects on the two parties. if one of the partners in an association is much larger than the other, it is generally known as the host. in parasitism, the parasite benefits at the host's expense. in commensalism, the two live together without harming each other, while in mutualism, both parties benefit. most parasites are only parasitic for part of their life cycle. by comparing parasites with their closest free-living relatives, parasitism has been shown to have evolved on at least 233 separate occasions. some organisms live in close association with a host and only become parasitic when environmental conditions deteriorate. a parasite may have a long-term relationship with its host, as is the case with all endoparasites. the guest seeks out the host and obtains food or another service from it, but does not usually kill it. in contrast, a parasitoid spends a large part of its life within or on a single host, ultimately causing the host's death, with some of the strategies involved verging on predation. generally, the host is kept alive until the parasitoid is fully grown and ready to pass on to its next life stage. a guest's relationship with its host may be intermittent or temporary, perhaps associated with multiple hosts, making the relationship equivalent to the herbivory of a wild-living animal. another possibility is that the host–guest relationship may have no permanent physical contact, as in the brood parasitism of the cuckoo. hosts to parasites micropredator, parasite, parasitoid, and predator strategies compared. their interactions with their hosts form a continuum. micropredation and parasitoidism are now considered to be evolutionary strategies within parasitism.