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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.
Yellow fever is a viral illness spread to humans through the Aedes aegypti mosquito, a type of mosquito that thrives in subtropical and tropical areas of Africa and South America. The virus is spread from primate (monkey or human) to primate through infected mosquitos.