Parasitism

Parasitism is a close relationship between two species in which the parasite lives on or inside its host, causing it some degree of harm, and is structurally adapted to that way of life. Its major strategies include parasitic castration, which destroys a host's ability to reproduce; direct transmission, requiring no intermediate carrier, as with lice and mites; trophic transmission, spread when one animal is eaten by another, as with tapeworms; vector transmission, relying on a carrier such as a mosquito; parasitoidism, in which the parasite eventually kills its host; and micropredation, in which a parasite feeds intermittently on multiple hosts. Parasitism differs from predation because, apart from parasitoids, parasites are usually much smaller than their hosts, rarely kill them outright, and often live in or on them for an extended time, and it differs from mutualism and commensalism in that the host is specifically harmed, though long-term coevolution can sometimes shift a parasitic relationship toward a more mutualistic one.