I always thought it was a little weird that the deadly germ warfare between the old-worlders and new-worlders was one-way. When Columbus brought over a host of diseases and the natives here died off in massive numbers, why didn't the same happen to the Europeans? Europe had its neighbors and exposure to various bugs was a part of life (making their immune systems stronger), but natives had their neighbors as well.
Now it seems, according to a study written up on SciAm.com that the new world sent back its own superbugs to Europe. Columbus, or one of his crew, seems to have been the one to introduce syphilis to Europe. The first outbreak of that disease in Europe was in 1495. There are 26 strains of the family to which syphilis belongs, some are non-sexually transmitted.
SCIAM
The syphilis bacteria: