Parasite vs. Host

Parasite

In biology, parasitism is a relationship between species, where one organism, the parasite, lives on or in another organism, the host, causing it some harm, and is adapted structurally to this way of life. The entomologist E. O. Wilson has characterised parasites as “predators that eat prey in units of less than one”. Parasites include protozoa such as the agents of malaria, sleeping sickness, and amoebic dysentery; animals such as hookworms, lice, and mosquitoes; plants such as mistletoe and dodder; and fungi such as honey fungus and ringworm. There are six major evolutionary strategies within parasitism, namely parasitic castrator, directly transmitted parasite, trophically transmitted parasite, vector-transmitted parasite, parasitoid, and micropredator.

Unlike predators, parasites, with the exception of parasitoids, 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. Parasitism is a type of consumer-resource interaction. Parasites of animals 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.

Parasites reduce host biological fitness by general or specialized pathology, from parasitic castration and impairment of secondary sex characteristics to modification of host behavior. Parasites increase their own fitness by exploiting hosts for resources necessary for their survival, in particular transmission. 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.

People have known about parasites such as roundworms and tapeworms since ancient Egypt, Greece, and Rome. In Early Modern times, Antonie van Leeuwenhoek observed Giardia lamblia in his microscope in 1681, while Francesco Redi described endo- and ectoparasites including sheep liver fluke and ticks. Modern parasitology developed in the 19th century. In human culture, parasitism has negative connotations. These were exploited to satirical effect in Jonathan Swift’s 1733 poem “On Poetry: A Rhapsody”, comparing poets to hyperparasitical “vermin”. In fiction, Bram Stoker’s 1897 Gothic horror novel Dracula and its many later adaptations featured a blood-drinking parasite. Ridley Scott’s 1979 film Alien was one of many works of science fiction to feature a terrifying parasitic alien species.