Parasitism is one version of
symbiosis ("living together"), a phenomenon in which two organisms which are
phylogenetically unrelated co-exist over a prolonged period of time, usually the lifetime of one of the individuals. The requirement for a prolonged interaction precludes predatory or episodic interactions (such as a
mosquito feeding on a host), which are usually not seen as symbiotic relationships. Symbiosis encompasses
commensalism ("eating at the same table", wherein two organisms co-exist in the same space, and one organism benefits while neither harming nor helping the other), through
mutualism (wherein both species benefit from the interaction) to parasitism, wherein one organism, usually physically smaller of the two (the
parasite) benefits and the other (the
host) is harmed. (Various forms of "social parasitism", kleptoparasitism, and "cheating parasitism", as discussed below, are characterized by a less close association between a parasite and a host, however.) Especially in the field of medical parasitology, the term "parasite" has come to mean a
eukaryotic,
pathogenic organism. Thus,
protozoan and
metazoan infectious agents are classified as parasites while
bacteria and
viruses are not. Interestingly,
fungi are not discussed in textbooks of medical parasitology, even though they are eukaryotic.
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