Parasite, organism that lives in or on a second organism, called a host, usually causing it some harm. A parasite is generally smaller than the host and of a different species.
Parasites are dependent on the host for some or all of their nourishment.
For example, a tapeworm, a flattened worm that lives in the gastrointestinal tract of mammals, lacks an intestine of its own and must absorb predigested food from the intestine of its host. This food is the tapeworm's only energy source for growth and reproduction. Parasitism affects most life forms, from bacteria infected by the viruses known as bacteriophages, to humans, who are subject to more than 1000 parasites known to cause disease. The acidity promotes fungal problems and tissue oxygen deprivation. They damage tissue directly and can obstruct organ function.