Parasitism describes a relationship between two species, a parasite and its host, in which the parasite benefits, while the host is harmed. Parasitism is one form of symbiosis, which more generally describes any situation involving a close relationship between organisms of different species.

Parasites are different from predators and parasitoids (which also derive benefits from certain interspecific interactions while harming the other participant) in that the host of a parasite is not necessarily killed. Instead, parasites derive benefits from their hosts, most often nutritional resources and shelter, over a longer period of time. It is in fact advantageous to parasites if they do not harm their hosts too badly, because that prolongs the period during which parasites can obtain benefits from hosts. However, in some cases, the impact of parasites on a host is great enough to cause disease, and in extreme cases, the death of the host...