A parasite is any living organism that uses another living organism as an edible hotel.
They rule our world with neither brains nor eyes. Rule from sunless thrones of flesh in pulsing, breathing castles that live and love and read Cracked.com oblivious to the primordial universe within their own tender, moist viscera.
Parasitic lifeforms - from microscopic protozoa to thirty-foot intestinal worms - can be found lurking in each and every non-parasitic or "free living" lifeform on our planet, easily outnumbering them in both total individuals as well as total species. Once thought to be lowly and primitive on the evolutionary chain of command, parasites are now thought to have an unrivaled influence on the forces of natural selection, manipulating predator-prey relationships and directly affecting which species thrive or perish.
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Whether a tiny mite drilling under the flesh of an armadillo or the gelatinous flatworm drinking from the gallbladder of a stingray, nothing can escape the slimy clutches of parasitism. One species of mite colonizes the left ear (only the left) of one species of moth. A species of parasitic fish inhabits the anus of a sea cucumber. Even parasites can have smaller parasites of their own. If it has a hole, nature has custom-designed something to invade that hole and feast on the unprotected tissues therein. If it doesn't have a hole, that's what corrosive acid is for.
A creature that lives in or on another creature, feeds off its tissues or fluids and offers no immediate benefit in return is a parasite in the most traditional sense. This relationship can often be quite simple, especially in the case of ectoparasitism (creepy-crawlies on the outside of your body) where such creatures as fleas, lice, ticks or leeches simply hitch a ride, dine and ditch. In the case of endoparasitism (heebie-jeebies in your innards) things tend to get a great deal more involved, as the creatures must figure out a way to spread from one host to another and protect themselves from a body's natural defenses.
Many endoparasites have incredibly complicated life cycles, passing from one host to another and taking on a completely different form at every turn. While some of these simply ride their host's natural body functions, others have the power to directly manipulate the behavior or even anatomy of the host animal, transforming simple-minded creatures into custom zombie slaves. Parasites can reprogram how a spider spins its web, convince a mouse to find and agitate the nearest hungry cat or restructure the body of a male crab to function like a female.