Hi Aiontay,
The Western Hemisphere has all sorts of indigenous diseases. What was missing in Beringia wasn't diseases, it was the ability to spread quickly. If someone in a hunter-gatherer band falls sick (say, he picked up influenza from a migratory duck he shot for dinner) then the worst that could happen is that he and his family would die. Others would be unaffected, and a potentially lethal epidemic would fizzle. That kind of disease screen showed up all over the world, including Polynesia, Australia, and so forth. Diseases hit hard there too.
As for why the Norse didn't bring anything with them, I think Pallas might be onto something, in that they happened to voyage at a time when there weren't massive epidemics scudding around. So far as I know, the Black Death made it into Scandanavia, and, well, we all suffer from influenza in cold weather. Cold isn't necessarily a barrier to disease.
F
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