What made you want to look up mean? Please tell us where you read or heard it (including the quote, if possible).
Malaria - and its cousin, yellow fever - had significant implications during the war and the debilitating consequences of the disease were already certain before the Civil War had started in earnest and well before its final history had been written.
Mosquitoes spread yellow fever. Yellow fever can cause headaches, chills and fever. In severe cases of yellow fever the liver can fail. It is advisable to wear insect repellent when traveling to areas where yellow fever is endemic. Yellow fever can cause headaches, chills and fever.