Correct me but if I remember correctly the sailors that went back to Europe took a lot of Indian sexual diseases with them then did not exist in europe before.
I'm not sure of that angle, but it wouldn't suprise me in the least. Perhaps the Europeans had more resistance than the Indian's, who were unable to fight even basic european diseases that people had built up a resistance to over the centuries. IOW, something that would sicken a european for a week or two would kill them in days.
It's more complicated than that. Apparently there were two strains floating around, a European one and an American one. Here's more information:
http://www.pbs.org/wnet/secrets/case_syphilis/index.html