The Sea Wasp (Ryk E. Spoor)

The Norse, especially the folks from Greenland and Iceland that founded Vinland were pretty isolated themselves. They didn't have the population density to support a lot of the nastier diseases, plus, none of them had active cases when they got there.

But one of Cortez's men had smallpox. Likewise other folks later had things like measles and a host of others.

With Cortez, smallpox went from his man to one of the more densely populated areas in the Americas. And spread like wildfire.