Population density. If you had a nasty disease emerge in the New World, it would be as devastating to the local population at first as it would be to a later colonizing European. If something that nasty gets loose in a population like the Native Americans, you end up with so few survivors that the group/tribe is wiped out and the disease fades away. The issue is that if disease "X" kills a high percentage of the population, if it hits a large population the absolute number of survivors is enough to carry on, in a small popualtion with the same percentage die-off you're screwed. This scenario is what happened to a lot of native American societies/tribes when European Disease hit them - there weren't enough survivors to care for those who needed care (like young children who survived) or to farm/hunt/make clothes etc and the survivors succumbed to other causes of mortality or were absorbed in other tribes and the original culture vanished.
The Mongols do not represent a low density population originating a disease. The were a vector conveying the disease from dense China to the dense west.
The Mongols do not represent a low density population originating a disease. The were a vector conveying the disease from dense China to the dense west.