Parasite

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A parasite is an organism that spends a significant portion of its life in or on the living tissue of a host organism and which causes harm to the host without immediately killing it. Parasites also commonly show highly specialized adaptations allowing them to exploit host resources.

The harm caused to a host by a parasite can take many forms, from direct pathology, including various specialized types of tissue damage, such as castration, to more subtle effects such as modification of host behaviour. Many cuckoos, for example, are brood parasites that use their unwitting hosts as lifetime "babysitters": cuckoo young are raised by adults of the host species, but adult cuckoos fend for themselves.

The biological interaction between host and parasite is considered by some to be a type of symbiosis. Classically, the distinction between parasites and other symbionts was methodological: parasites were symbionts that could not be kept alive outside the host, unlike bacteria, for example, which could be cultured in a laboratory.

Evolutionarily speaking, biotrophic parasitism is an extremely successful mode of life. Depending on the definition used, as many as half of all animals have at least one parasitic phase in their life cycles, and it is also frequent in plants and fungi. Moreover, almost all free-living animals are host to one or more parasite taxa.

Necrotrophs are parasites that use another organism's tissue for their own nutritional benefit until the host dies from loss of needed tissues or nutrients. Biotrophs' hosts are kept alive, and parasites in a symbiotic relation which is a co-operative relationship between two species for mutual gain.