Parasitism
relationship between species, where one organism, the parasite, lives on or in another organism, the host, causing it some harm, and is adapted structurally to this way of life / From Wikipedia, the free encyclopedia
Parasitism is a form of one-sided symbiosis.[1][2] 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.[3] 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: