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Parasitism is a type of symbiotic relationship between organisms of different species where one organism, the parasite, benefits at the expense of the host.

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 to the modification of host behaviour.

Parasites increase their fitness by exploiting hosts for food, habitat and dispersal.

Host defenses also evolve in response to attacks by parasites.

Theoretically, parasites may have an advantage in this evolutionary arms race because of their more rapid generation time. Hosts reproduce less quickly than parasites, and therefore have fewer chances to adapt than their parasites do over a given span of time.