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.
The yellowing occurs because the disease causes liver damage, hepatitis. For some people, yellow fever has no initial symptoms, while for others, the first symptoms appear from three to six days after exposure to the virus from a mosquito bite. An infection with yellow fever typically has three phases.