Unlike commensalism and mutualism, the parasitic relationship harms the host, either feeding on it or, as in the case of intestinal parasites, consuming some of its food. Unlike saprotrophs, parasites feed on living hosts, though some parasitic fungi, for instance, may continue to feed on hosts they have killed. Parasitism is a kind of symbiosis, a close and persistent long-term biological interaction between a parasite and its host. Head (scolex) of tapeworm Taenia solium, an intestinal parasite, has hooks and suckers to attach to its host The related term parasitism appears in English from 1611.
įirst used in English in 1539, the word parasite comes from the Medieval French parasite, from the Latin parasitus, the latinisation of the Greek παράσιτος ( parasitos), "one who eats at the table of another" and that from παρά ( para), "beside, by" + σῖτος ( sitos), "wheat", hence "food". Ridley Scott's 1979 film Alien was one of many works of science fiction to feature a parasitic alien species. In fiction, Bram Stoker's 1897 Gothic horror novel Dracula and its many later adaptations featured a blood-drinking parasite. These were exploited to satirical effect in Jonathan Swift's 1733 poem "On Poetry: A Rhapsody", comparing poets to hyperparasitical "vermin". In human culture, parasitism has negative connotations. Modern parasitology developed in the 19th century. In early modern times, Antonie van Leeuwenhoek observed Giardia lamblia in his microscope in 1681, while Francesco Redi described internal and external parasites including sheep liver fluke and ticks. People have known about parasites such as roundworms and tapeworms since ancient Egypt, Greece, and Rome. Although parasitism is often unambiguous, it is part of a spectrum of interactions between species, grading via parasitoidism into predation, through evolution into mutualism, and in some fungi, shading into being saprophytic. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular by feeding on them and by using intermediate (secondary) hosts to assist in their transmission from one definitive (primary) host to another. Parasites reduce host fitness by general or specialised pathology, from parasitic castration to modification of host behaviour. Classic examples include interactions between vertebrate hosts and tapeworms, flukes, the malaria-causing Plasmodium species, and fleas. Parasites of animals are highly specialised, and reproduce at a faster rate than their hosts. Like predation, parasitism is a type of consumer–resource interaction, but unlike predators, parasites, with the exception of parasitoids, are typically much smaller than their hosts, do not kill them, and often live in or on their hosts for an extended period. One major axis of classification concerns invasiveness: an endoparasite lives inside the host's body an ectoparasite lives outside, on the host's surface. There are six major parasitic strategies of exploitation of animal hosts, namely parasitic castration, directly transmitted parasitism (by contact), trophically transmitted parasitism (by being eaten), vector-transmitted parasitism, parasitoidism, and micropredation. Parasites include single-celled protozoans such as the agents of malaria, sleeping sickness, and amoebic dysentery animals such as hookworms, lice, mosquitoes, and vampire bats fungi such as honey fungus and the agents of ringworm and plants such as mistletoe, dodder, and the broomrapes. Wilson has characterised parasites as "predators that eat prey in units of less than one". Parasitism is a close relationship between species, where one organism, the parasite, lives on or inside another organism, the host, causing it some harm, and is adapted structurally to this way of life. A fish parasite, the isopod Cymothoa exigua, replacing the tongue of a Lithognathus
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