A form of symbiosis in which one organism (called parasite) benefits at the expense of another organism usually of different species (called host); the association may also lead to the injury of the host
Symbiosis is a close, long-term association between organisms of different species. It may be in the form of mutualism wherein the relationship between the two organisms is interdependent. It may also be in the form of commensalism wherein an organism benefits from the association while the other stays generally unharmed. Symbiosis may also be in the form of parasitism. In parasitism, the organism benefits at the expense of another organism. The organism that benefits from the association is called a parasite whereas the one that does not benefit but rather harmed is called a host. Thus, in parasitism the symbiotic relationship is non-mutual. However, the parasitic organism does not necessarily kill its host. This makes parasitism different from predation wherein the predator eventually kills the prey. Furthermore, the parasite is generally smaller than the host. Some of the parasites would also need a vector to aid in their transmission to their host.
Examples of parasitism are the helminthes (worms) in the intestines of the host, lice (Pediculus humanus capitis) in human head, Plasmodium species transmitted by anopheline mosquito and causing malaria in humans.