We’re all aware that smallpox went one way in the Colombian exchange, syphilis the other (no, don’t tell me it didn’t). Populations entirely unused or exposed to either died in their droves.
We also know that populations free of measles (say, Faroes) die in their drifts when it finally arrives.
So, colds and ‘flus. Making the distinction between the two. Clearly these have been around a long time. But then so also have human populations been very split for a very long time. So, did the European cold kill lots of Americans? The European ‘flu? The American versions lots of Europeans?
A case wouldn’t have survived the length of the early voyages and getting variolation to do so for smallpox required significant planning. But at some point the American pop would have been exposed to these common European diseases and vice versa. What then happened?
My assumption is that two different effects happened. One, that old diseases are, as these things go, less virulent. But also, old diseases in one population can hit a population that’s never been exposed to a close variant and so has much less immunity. What actually happens then depends upon the size of the two effects. Is a cold so lightweight that not much, or is it so different from the separately evolved one over 13,000 years that a lot happens?