Malaria is a parasitic disease that causes between 1 million and 3 million deaths each year, mainly African children. Three billion persons—close to 50% of the world's population—live in 107 countries and territories in which malaria is endemic. Most mortality is due to Plasmodium falciparum, a protozoan parasite transmitted by the Anopheles mosquito, which is responsible for more than 515 million cases of disease annually: In addition, almost 5 billion febrile episodes resembling malaria, but which cannot be definitively identified as such, occur in endemic areas annually. The medical, epidemiologic, and economic burdens due to malaria have greatly impeded development in endemic countries, particularly in Africa.