[ikurei]

(Not an expert, I just happen to have read a fair bit about the topic, grain of salt highly recommended)

Cities. Very deadly pandemics generally come from diseases that jump from another species to humans.

It is not advantageous for a virus to have a very high lethality, for any parasite there is no advantage in killing large numbers of your host. It's the diseases that are kind of new to the host that can be more lethal.

This jump of a disease from one species to another happens more easily when there is a lot of close contact between them. Large cities where humans lived with animals, and in not-very sanitary conditions, were a perfect ground for this.

In America cities appear to be less dense, and animal husbandry was way less extended (AFAIK, llamas and alpacas are the only animals domesticated in America).