There has long been suspicion that when Christopher Columbus returned to Europe, carrying plenty of intriguing spoils, he also took back a venereal disease that did plenty of spoiling of its own. Syphilis ravaged the Old World in the centuries after Columbus sailed, with the first epidemic occurring around 1495 soon after he went back to Spain following his first voyage. In 2008, researchers at Emory University published an article in which they had studied 26 strains of bacteria related to syphilis. They determined that the strains currently causing venereal syphilis are closely related to what caused an isolated infection centuries ago in South America. Essentially, their work showed that syphilis as we know it more likely came from the New World to the Old rather than the other way around, and Columbus is the likeliest carrier.
The most popular counterargument is that skulls showing signs of syphilis and dating to pre-Columbian times were found in England, which does give Columbus a possible out. And it's worth noting that while the Europeans might have taken horses and gunpowder to the New World, they also carried devastating diseases of their own, including smallpox, typhus, cholera and the measles.