A disease caused by members of the protozoan genus Plasmodium, a widespread group of sporozoans that parasitize the human liver and red blood cells. Four species can infect humans: P. vivax, causing vivax or benign tertian malaria; P. ovale, a very similar form found chiefly in central Africa that causes ovale malaria; P. malariae, which causes malariae or quartan malaria; and P. falciparum, the highly pathogenic causative organism of falciparum or malignant tertian malaria. Malaria is characterized by periodic chills, fever, and sweats, often leading to severe anemia, an enlarged spleen, and other complications that may result in loss of life, especially among infants whose deaths are almost always attributed to falciparum malaria. The infective agents are inoculated into the human bloodstream by the bite of an infected female Anopheles mosquito, more than 60 species of which can carry the infection to humans. The disease is found in all tropical and some temperate regions, but it has been eradicated in North America, Europe, and Russia. Despite control efforts, malaria has probably been the greatest single killer disease throughout human history and continues to be a major infectious disease. An estimated 1 million children die of malaria in Africa each year, and estimates of the world's malaria death toll vary from 1.5 to 2 million cases annually.