My understanding is that the leading hypothesis is that: 1. The “Old World” had a much more extensive animal husbandry. A lot of the nastiest diseases human be

My understanding is that the leading hypothesis is that: 1. The “Old World” had a much more extensive animal husbandry. A lot of the nastiest diseases human beings pick up come from animal reservoirs. With humans living in closer quarters to animals for a much longer period of time in Eurasia/Africa than the Americas, more diseases crossed over and became endemic among the population. 2. A great amount of the Eurasian continent is along an East-West axis, whereas the Americas are primarily North-South. This means that seasons and climates and climates tended to be more closely aligned, which leads to easier spread of agriculture and domesticated animals, which leads to faster population growth and disease spread.