Parasitism is a non-mutual symbiotic relationship amongst species, where one species, the parasite, is benefitted at the cost of the other, which is the host. Usually parasite (in biological usage) a term attributed primarily to organisms visible to the naked eye, or macro parasites (like helminths). It is an association amongst two non identical organisms where one benefits at the expense of the other. All infectious agents causing illness come in this category. Examples of parasitism are the helminthes (worms) that infect the intestines of the host, lice (Pediculus humanus capitis) in human head, Plasmodium species transmitted by anopheline mosquito and causing malaria in humans.