Parasitism

Parasitism is a non-mutual 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 needed more than one host (e.g. Taenia solium). These are now called macroparasites (typically protozoa and helminths). Parasite now also refers to microparasites, which are typically smaller, such as viruses and bacteria, and can be directly transmitted between hosts of the same species. Examples of parasites include the plants mistletoe and cuscuta, and organisms such as hookworms.

Unlike predators, parasites are generally much smaller than their host; both are special cases of consumer-resource interactions. Parasites show a high degree of specialization, 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 by the fact that parasitoids generally kill their hosts. Parasitoidism occurs in a similar variety of organisms to that in which parasitism occurs.

Parasites reduce host biological fitness by general or specialized pathology, such as parasitic castration and impairment of secondary sex characteristics, to the modification of host behaviour. Parasites increase their fitness by exploiting hosts for resources necessary for their survival, e.g. food, water, heat, habitat, and transmission.

Although parasitism applies unambiguously to many cases, it is 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.