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: