Eurasia was a cesspool. The Americas were not.<p>Eurasians/Africans evolved an immune system capable of fighting infectious diseases because for thousands of years they lived in close quarters with domesticated animals and other people.<p>American Indians had few domesticated animals (only Llamas and Alpacas, I think), minimal over overcrowding and hence had few infectious disease. This caused them to be tall and healthy by contemporary Eurasian standards, but also made them prone to illness.<p>For much the same reason, I got food poisoning twice in two weeks in Egypt but my (Asian) Indian girlfriend came away unscathed.