I have always been taught that one of the reasons the Spanish conquered the South American peoples was because of European diseases decimating the native populations. If true, then why weren't the Spanish similarly decimated by foreign South American diseases?
This and similar questions are asked almost constantly. Please read these entries.
There is some speculation that Syphilis was carried to Europe from the Americas by European sailors, but there is no consensus. Syphilis was present in the Americas before European contact, but it is difficult to determine if it was present elsewhere at the same time.