Parasitism

Parasitism is an interspecific interaction in which one species, the parasite, obtains its nourishment by living on or within the body of another species, the host, at the host's expense -- represented as (+, −). Unlike a predator, a parasite usually does not kill its host outright (at least not immediately), because a parasite that quickly kills every host it infects would rapidly run out of hosts to exploit; instead, the relationship is typically a more prolonged, one-sided drain on the host's resources and health.

Parasites are broadly grouped into two categories based on where they live relative to the host's body. Ectoparasites live on the outer surface of the host's body; well-known examples affecting humans include lice (Pediculus), which attach to hair and feed on blood from the scalp, and ticks and mosquitoes, which attach briefly to feed on blood. Endoparasites live inside the host's body, often within a specific organ or tissue; examples include the roundworm Ascaris and various species of tapeworm living in the human intestine, the malarial parasite Plasmodium living inside human red blood cells and liver cells, and the liver fluke living within the host's bile duct.

Because a fully parasitic way of life is so different from a free-living one, parasites have evolved a distinctive suite of adaptations. Many parasites show a loss of unnecessary sense organs, since a permanent internal parasite has little use for eyes once it is safely inside a host's gut. Most possess specialised adhesive organs, suckers, or hooks that let them cling firmly to host tissue and resist being dislodged by the host's own movements or digestive processes. Intestinal tapeworms show an extreme adaptation in that they have lost their own digestive system entirely, since they are bathed in the host's already-digested food and can absorb nutrients directly across their body surface. Parasites also typically show a very high reproductive capacity, producing enormous numbers of eggs, because the odds of any single egg successfully finding and infecting a new host are extremely low, and this loss must be compensated for by sheer numbers. Many parasites additionally have complex life cycles involving one or more intermediate hosts -- the malarial parasite, for instance, must pass through both a human host and a female Anopheles mosquito to complete its life cycle, and the liver fluke requires a snail as an intermediate host. …