I realize this is an older column, but everything I’ve read and the documentaries I’ve seen stated that the Europeans brought smallpox and took back syphilis - specifically to Spain.
That was the common belief, but I think more recent evidence points to syphilis being found in Europe well before Columbus. No cites handy; I saw a PBS show about it, though.
I, for one, suspect that there were New World and Old World strains to which the local humans had considerable immunity, but which burned through the opposite populations.
For about the last decade, the standard evidence for pre-Columbian syphillis in Europe has been from an excavation in Hull and that has been questioned.
As I’ve suggested before on the Dope, I suspect the archaeological uncertainties are too great in this instance for a definite answer. Is the pre-Columbian evidence in the Americas actually better than that in Europe? And then John’s suggestion seems as plausible to me as any other.
Cecil addresses this fairly quickly in a 15-year old column: Why did so many Native Americans die of European diseases but not vice versa? - The Straight Dope