American diseases wouldn't consistently do so either. Only the initial voyages would suffer high death rates, after that people would be immune. It might be written up to the first expeditions having bad luck.Malaria isn't wiping out the majority of the ship's crew, however.
Well, for my little extrapolation I postulated a 70% death rate.Really, if you want a second Black Death that's one thing. If you want a 95% die off, Europeans are going to have to be utter morons.
Wikipedia tells me the Black Death killed between 1/3-1/2 of Europeans, so a 70% death rate is maybe two Black Death equivalents. They could both have come back with initial forays.