[soapdog]

There were diseases in the Americas, they were deadly too. One thing to consider is how they spread and if they are contagious between humans. Malaria is horrible, but you don't catch it from other people, pox on the other hand will spread between humans. This is just one example. Being mostly exposed to nasty diseases that are not contagious will lead a culture to have different behaviour when dealing with the sick. These practices will often involve taking close care of the sick. Such practices will prove horrible for dealing with highly lethal contagious diseases from Europe.