Deaths due to diseases spread from Europeans to native Americans, when conquistadors first discovered and inhabited the Americas, were insurmountable. Almost 9/10 died. My question is this: Why were there no diseases that affected the Europeans as badly as the ones that affected the native Americans?
Not to discourage further answers to this specific question, because there's always more that can be said, but there's a great answer by /u/anthropology_nerd about the diseases from the Americas that affected European colonisers here.