One of the largest fields in parasitology, medical parasitology is the subject which deals with the parasites that infect man, the diseases caused by them, clinical piture and the response generated by man against them. It's also concerned with the various methods of their diagnosis, treatment and finally their prevention & control. A parasite is an organism that live on or within another organism called the host . These include organisms such as:? Plasmodium spp., the protozoan parasite which causes malaria. The four species of malaria parasites infective to humans are Plasmodium falciparum,Plasmodium malariae, Plasmodium vivax & Plasmodium ovale. ? Leishmania donovani, the unicellular organism which causes leishmaniasis ? multicellular organisms such as Schistosoma spp., Wuchereria bancrofti and Necator americanus etc.Parasitology is the study of parasites and as such does not include bacterial, fungal or viral parasites. Human parasites are separated into intestinal and blood borne parasites. For a parasite to be defined as intestinal it must have an intestinal life cycle stage, though it may have life-cycle stages in the heart, circulation, lung, tissue, other animals.