The first recorded instance of syphilis in Europe was made in 1494 when it ravaged the French troops that were besieging the city-state of Naples. It is thought that they … ah, shared the favors of some local ladies … with their Spanish allies who joined them for the seige. The Spanish got it from Columbus’ returning sailors, who got it from the Americas.
Actually, the origin of syphilis is hotly debated. But the theory I just explained is the “traditional” explanation. If the theory is true, syphilis was the only New World disease brought to the Old World. It wasn’t exactly a fair exchange: measles, smallpox, and cholera are just the more famous of innumerable other plagues Europeans and Africans accidentally brought across the Atlantic.