Ectoparasitism

In biology, parasitism is a non-mutual relationship between species, where one species, the parasite, benefits at the expense of the other, the host. Traditionally parasite primarily meant an organism visible to the naked eye, or a macroparasite (such as a helminth). Microparasites are typically far smaller, such as protozoa,viruses, and bacteria. Examples of parasites include the plants mistletoe and cuscuta, and animals such as hookworms.

Unlike predators, parasites typically do not kill their host, are generally much smaller than their host, and often live in or on their host for an extended period. 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 include interactions between vertebrate hosts and tapeworms, flukes, the Plasmodium species, and fleas. Parasitoidy is an evolutionary strategy within parasitism in which the parasite eventually kills its host.

Parasites reduce host biological fitness by general or specialized pathology, from parasitic castration and impairment of secondary sex characteristics, to the modification of host behavior. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular transmission. Although parasitism often applies unambiguously, it is part of a continuum of types of interactions between species, grading via parasitoidy into predation, through evolution into mutualism, and in some fungi, shading into being saprophytic.

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