Yeah, this makes sense. I was thinking like... what if one Norseman was really, *really* sick, and just brought over all kinds of things, but your points make more sense. Hell, one of the more interesting outcomes (surviving Incan empire) may not even play out as I want it to, since there's no guarantee the diseases spread that far South even in 500 years thanks to limited trade links.This gets brought up a lot but it isn't really plausible to the degree you're imagining given the way epidemic disease works. Basically, Iceland and Greenland were fairly free of disease (including smallpox) until the end of the Middle Ages. Further, the long travel time to the New World acts as a natural quarantine for all but a few diseases (ones with long incubation period plus long period of infectiveness) so in the end I don't think you'd get much more than chickenpox/shingles, mumps, and whooping cough transmitted by humans. Further, you have the factor that because of relative lack of trade and lack of dense population, the disease wouldn't become endemic like smallpox was in Europe and Africa. We see this in remote parts of the Old World, like in Siberia and certain parts of Africa, the people suffered just as heavily from epidemic as New World natives did.
That said, just a few epidemic diseases in the New World might have huge consequences since those would be removed from the long list of plagues Europeans brought and societies would have a better social response to disease.
Ah well. Thanks anyway!