Jared Diamond considered the disastrous impact of Eurasian and African diseases on the inhabitants of the New World, contrasted with a much smaller impact in the opposite direction, and concluded that a major factor had probably been transmission from domesticated animals. Eurasians domesticated quite a few animals, Amerindians not many – perhaps that was the explanation. In Guns, Germs, and Steel (p 207), he mentions measles, tuberculosis, smallpox, influenza, pertussis (whooping cough), and falciparum malaria as likely cases of transmission from domesticated animals.
We know a lot more about this we did twenty years ago, since we’ve been sequencing the genes of everything in sight – and it appears that Diamond was mistaken about the most important members of that list. TB appears to be ancient in humans, smallpox probably came from some East African rodent, while falciparum malaria seems to have derived from a form of malaria carried by gorillas. Measles really does descend from rinderpest, a cattle plague, but then rinderpest (and mumps) probably descend from bat viruses. Domesticated animals do play a role in influenza, along with wild birds. I don’t think we know the origins of pertussis.
So why then was the Old World such a fount of infectious disease? Well, it’s bigger. Civilization was older, had had more time to pick up crowd diseases. Important pathogens, especially those with insect vectors like malaria, maybe couldn’t make it through Beringia. Transportation and trade were more advanced in the Old World, and spread disease more efficiently.
I don’t think that Diamond was making excuses for Amerindians in this, as he was when talking about domestication itself: having lots of plagues isn’t usually considered an accomplishment. It seemed like a reasonable idea at the time, considering the state of the art. It seemed so to others as well, like William McNeill. It’s not totally wrong – definitely true for measles – but it’s not a huge part of the explanation.