When I give public talks, a very common question from the audience is "how come the Native Americans died of European diseases, but the Europeans didn't die from Native American diseases?"
It's a good question. It's a question that never has any real relation to my talks, but I am always glad to get it because it shows that the audience is intelligent and has been thinking about what looks like a puzzling matter. After all, shouldn't it go both ways - if a small number of European settlers, priests, and soldiers can inadvertently wipe out huge swaths of the population with communicable disease to which the natives are not immune, doesn't it follow that the colonists would likewise be vulnerable to native pathogens?
The simple fact of the matter is that nobody really knows why native pathogens (bacteria and viruses) didn't do much harm to the European colonists (or the African slaves that they would eventually bring, or the Asian settlers who would come later still), but we are not completely clueless either*. Most explanations are based on our knowledge of how pathogens spread through a population. Most Native American communities were less dense than those of Europe or Asia, and parts of Africa. This may have provided less opportunity for pathogens to evolve, and may simply mean that there were less, and less virulent, diseases in the Americas prior to the arrival of Europeans.
A different, though not mutually exclusive, explanation is that people of the Old World (Europe, Africa, Asia) had long lived in or near cities, and as a result evolution had produced stronger immune systems - those who are more vulnerable to illness typically died at a young age and consequently left no offspring. Likewise, the large number of people living in poor sanitary conditions within the cities and towns of the pre-modern and early modern Old World would have been exposed to a wide range of pathogens, potentially providing immune systems primed for partial protection against new infections.
A new study has shown evidence that people descended from populations with long histories of living in cities (such as Italians, who picked up city living before Rome) tend to have genetic traits that improve the immune response to infection, and people descended from non-urban peoples tend to lack this trait. Note the "tend to" - the trait is present in some people descended from non-urban peoples, and lacking in some people who are descended from urbanites - it's a statistical difference and not a categorical one. Still it is interesting.
Although this is being interpreted as evidence that city living led to selection for stronger immune systems, there may be other explanations for the finding. Still, it's interesting new information to address this old question.
*However, it should be noted that there is evidence, far from conclusive but there nonetheless, that syphilis may have come from the Americas.