So, Europeans, Asians, and Africans seem to be somewhat protected from diseases with horrifically high kill rates purely by living on the largest landmass. But, this flies in the face of the logic that diseases become less deadly over time, not more. Hmmm. I guess this boils down to one question I have: Is a new disease likely to have a 90% kill rate upon first appearance in humans, or does it have to first inhabit a large number a people before being exposed to isolated people in order to be that destructive?
With very high kill rates you should not forget the mechanism of natural selection, in such cases the survivors probably had some kind of genetic advantages that gave them a better chance. So over the time the population would become more resistent against a certain mode of attack.
I don't know if the genetic make up of the native americans vs the aboriginals was of influence on the different lethality of diseases, but i could imagine it also played a role.