Based on Guns, Germs and Steel, Europeans had so many terrible diseases because of the close proximity to animals that carried the ancestors of those diseases:

Based on Guns, Germs and Steel, Europeans had so many terrible diseases because of the close proximity to animals that carried the ancestors of those diseases: pigs, cows, sheep, chickens and because they spread those diseases around much easier due to the geography. Native American (hemisphere) didn't have the animals to do the same thing (just some native turkies and ground mammals I think). If the first Native American ship had shown up in Europe and then gone home, the exact same diseases would have done the exact same thing.