Medical Parasitology deals with the study of parasites infecting humans. The diseases caused by them and the clinical manifestations produced in infected humans. It is also concerned with various methods of their diagnosis, treatment and their prevention and control. There are different types of parasites and hosts. Parasiteslivesonitshostforitsnourishment and survival. The relationship between host and the parasite can be symbiotic, commensalic or parasitic. Parasites of medical importance comes under the kingdom called Protista and Animalia. Protista includes the microscopic single - celled eukaryotes known as protozoa. In contrast, helminths are macroscopic, multicellular worms possessing well differentiated tissues and complex organs belonging to the kingdom Animalia. Protozoa includes Entamoeba and Giardia which cause intestinal infections (dysentery and diarrhoea) Leishmania donovani, the unicellular tissue flagellatis causes Leishmaniasis. Plasmodium spp., the protozoan parasite which causes malaria are transmitted by female Anopheles mosquito carring sporozoites forms of the parasite. The four species infective to humans are P. falciparum, P. malariae, P. vivax and P. ovale. Multicellular organisms and intestinal worms. The helminths such as Ascaris lumbricoides causes Ascariasis, an infection of the small intestine. Ascariasis is the most common roundworm infection.