"M" is for Malaria. Malaria was arguably the most significant disease in the history of South Carolina from the colonial period until the early 20th century. It is a parasitic infection caused by protozoa known as plasmodia and transmitted by anopheles mosquitoes. There are two types of disease: one introduced with European settlers in the 1670s and a more virulent form that came with the importation of large numbers of West Africans in the 18th century. During the 19th century malaria became a major health problem in much of the state, especially along river valleys. By the early 1950s the disease had virtually disappeared for reasons that are still not entirely understood—but improvements in mosquito control and improved nutrition probably played a part. The most common symptoms of malaria are fever, chills, and aches.