Infectious diseases from the Old World — smallpox, falciparum malaria, yellow fever, bubonic plague, cholera, measles, whooping cough, etc. — devastated the people of the New World. Montezuma’s Revenge took the form of two diseases — neither one a water-borne GI bug:
First, syphilis: the first known epidemic was in 1495, in Naples, during a French invasion. By 1520 it had reached Africa and China.
From the timing of the first epidemic, and the apparent newness of the disease, many have suspected that it was an import from the New World. Some, like Bartolome de las Casas, had direct knowledge: Las Casas was in Seville in 1493, his father and uncle sailed with Columbus on the second voyage, and he himself traveled to the New World in 1502, where he spent most of the rest of his life working with the Amerindians. Ruiz Diaz de Isla, a Spanish physician, reported treating some of Columbus’s crew for syphilis, and that he had observed its rapid spread in Barcelona.
I have seen someone object to this scenario, on the grounds that the two years after Columbus’s return surely couldn’t have been long enough to generate a major outbreak. I think maybe that guy doesn’t get out much. It has always looked plausible, considering paleopathological evidence (bone changes) and the timing of the first epidemic. Recent analysis shows that some American strains of pinta (a treponemal skin disease) are genetically closest to the venereal strains. I’d say the Colombian theory is pretty well established, at this point.
Interestingly, before the genetic evidence, this was one of the longest-running disputes among historians. As far as I can tell, part of the problem was (and is) that many in the social sciences routinely apply Ockham’s razor in reverse. Simple explanations are bad, even when they fit all the facts. You see this in medicine, too.
The story of tungiasis, chigger flea infestation, is simpler and clearer. Female chigger fleas burrow into the skin (often the big toe) and start releasing eggs, which can lead to infection, sometimes even to gangrene. It looks as if they originated in the West Indies, and were inadvertently spread to the mainland by the Spanish. Tungiasis hit the crew of the Santa Maria. Much later, the chigger flea was introduced to Africa, carried in ballast by a ship from Brazil: the Thomas Mitchell, in 1873. It spread all through tropical Africa, and made it to India and Pakistan in 1899. It causes quite a bit of trouble even today, particularly in poor areas — specifically, people without shoes.
Typhus and rheumatoid arthritis may also have New World origins.