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Synopsis

Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the other, the host. Traditionally parasite referred to organisms with lifestages that went beyond one host (e.g. Taenia solium), which are now called macroparasites (typically protozoa and helminths). Parasites can now also refer to microparasites, which are typically smaller, such as viruses and bacteria and can be directly transmitted between hosts of one species.

Unlike predators, parasites are generally much smaller than their host, although both are special cases of consumer-resource interactions. Parasite show a high degree of specialization for their mode of life, and reproduce at a faster rate than their hosts. Classic examples of parasitism include interactions between vertebrate hosts and diverse animals such as tapeworms, flukes, the Plasmodium species, and fleas.

Parasitism is differentiated from the parasitoid relationship, though not sharply, by the fact that parasitoids generally kill or sterilise their hosts. Parasitoidy occurs in about as many classes of organism as parasitism does.

The harm and benefit in parasitic interactions concern the biological fitness of the organisms involved. Parasites reduce host fitness in many ways, ranging from general or specialized pathology (such as parasitic castration), impairment of secondary sex characteristics, to the modification of host behaviour. Parasites increase their fitness by exploiting hosts for resources necessary for the parasite's survival: (i.e. food, water, heat, habitat, and dispersal).

Although the concept of parasitism applies unambiguously to many cases in nature, it is best considered part of a continuum of types of interactions between species, rather than an exclusive category. Particular interactions between species may satisfy some but not all parts of the definition. In many cases, it is difficult to demonstrate that the host is harmed. In others, there may be no apparent specialization on the part of the parasite, or the interaction between the organisms may be short-lived. In medicine, only eukaryotic organisms are considered parasites, with the exclusion of bacteria and viruses. Some branches of biology, however, regard members of these groups as parasitic.

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