Parasitism is a form of one-sided symbiosis. The parasites live off the host. They may, or may not, harm the host. Parasitoids, on the other hand, usually kill their hosts. A parasitic relationship is the opposite of a mutualistic relationship. Examples of parasites in humans include tapeworms and leeches. World-wide, the most serious cause of human death by a parasite is malaria.

A definition:

• A parasite is an organism drawing nutrients from a living host. It lives in or on another organism, getting from it part or all of its food. It usually shows some degree of adaptive modification, and causes some degree of damage to its host.