Malaria. Artwork of a malaria infection in the bloodstream. Malaria is a tropical disease caused by the protozoa Plasmodium sp. (crescents) being introduced into the blood by an infected mosquito bite. The plasmodia invade red blood cells, causing them to swell and burst, releasing thousands of tiny merozoites (purple) into the blood. The crescent forms of the plasmodia are the sex cells (gametocytes), which stay in the blood waiting for another mosquito to suck them up, restarting the cycle. The merozoites cause a high fever, and infect yet more red blood cells. Malaria is a serious disease, and is often fatal, Treatment is with quinine and its derivatives.