A parasite, originally, was someone who ate at another person's table
The Greek parasitos meant a dinner guest who fed at someone else's expense, a social insult long before it became biology. Today the word covers malaria protozoa, hookworms, lice, vampire bats, ringworm fungi and even mistletoe. E. O. Wilson summed them up neatly as predators that eat their prey in units of less than one.
Parasitism is a lasting close relationship in which one organism lives on or in another and harms it, with a body adapted for the job. Like predation it is a consumer taking from a resource, but most parasites are far smaller than their hosts, do not kill them and stay for a long time. Many reproduce faster than their hosts and specialise on a single host species, as with tapeworms, flukes and the Plasmodium organisms behind malaria.
Biologists classify strategies in overlapping ways. Endoparasites live inside the body, ectoparasites on its surface, and mesoparasites, including some copepods, wedge partly into an opening. Obligate parasites cannot complete their life cycle without a host, while facultative ones can. Direct life cycles use one host; indirect cycles pass through at least one intermediate host before reaching the definitive host, where sexual reproduction happens. Six broad ways of exploiting animals are recognised, from transmission by contact or by being eaten to parasitic castration and micropredation. Parasites can also alter host behaviour to aid their own spread.
The boundaries blur. Parasitism grades into predation through parasitoids, which do kill their hosts, can evolve toward mutualism, and in some fungi shades into feeding on dead matter.
People knew about roundworms and tapeworms in ancient Egypt, Greece and Rome. Antonie van Leeuwenhoek saw Giardia through his microscope in 1681, and Francesco Redi described sheep liver fluke and ticks, though parasitology as a discipline took shape only in the 19th century. The word entered English in 1539. Its negative sting has fuelled culture, from Jonathan Swift's 1733 satire likening poets to vermin to Bram Stoker's Dracula in 1897 and Ridley Scott's Alien in 1979.
Source: Parasitism
