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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 caused by a virus carried by mosquitoes. You can catch this disease if you are bitten by a mosquito infected with this virus. This disease is common in South America and in sub-Saharan Africa. Anyone can get yellow fever, but the elderly have a higher risk of severe infection.