In fact, this is potentially one of the reasons slavery was so prevalent/successful in the south and not the northern US - the line of north/south approximately lines up with the malarial zones, with the basic concept being that slavery in the south was necessary to run the farms and industry because white people were being wiped out but the africans were immune(r); but in the north, the whites weren't being malarially killed (as much). Source: "1493", http://www.amazon.com/1493-Uncovering-World-Columbus-Created/dp/0307265722