It's common knowledge that between the Americans and the Europeans, the Americans definitely took the worst of disease and plague, which resulted in a loss of life comparable to very few pandemics in human history. But what about the diseases indigenous to the Americas that Europeans had contracted, and even brought back to Europe?

Had European medicine and knowledge from previous plagues curtailed any potential devastating loss of life, or had there simply been no diseases that could have affected them in the same way the Americans had been affected, leaving the Europeans quite lucky in this exchange of unknowns?