Parasitism is a type of symbiotic relationship between two different organisms. The parasite benefits from a prolonged, close association with the host, which is harmed. In general, parasites are much smaller than their hosts, show a high degree of specialization for their mode of life and reproduce more quickly and in greater numbers than their hosts.

Classic examples of parasitism include the interactions between vertebrate hosts and such diverse animals as the tapeworms, flukes, Plasmodium species and scabs. 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 castration), impairment of secondary sex characteristics, to the modification of host behaviour.

Parasites increase their own fitness by exploiting hosts for food, 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, to the exclusion of bacteria and viruses.