To the answer above, I want to add that Europeans also contracted diseases among the indigenous population, one of the theories of the appearance of syphilis says that it was brought to Europe by the sailors of Columbus, who contracted it in Haiti.
Link to Wikipedia: ru .m.wikipedia.org
There are several reasons for this. First, there were simply more diseases in Europe. Many diseases (smallpox, flu, tuberculosis, malaria, plague, measles, cholera) were caught by humans from animals. In the New World, animal husbandry was much less developed (it seems that only llamas were domesticated). Secondly, due to the higher population density, European diseases did not disappear, but circulated for a long time throughout the population - while one person recovers, another manages to become infected. In small settlements in which, for example, North American Indians lived, the disease either disappeared or killed everyone. Therefore, European diseases could change and evolve. And the third reason - the most significant - since the inhabitants of the Old World faced diseases much more often, they developed immunity. We must not forget that the colonizers of the 17-18 centuries were the direct descendants of people who survived the era of terrible European epidemics.
All this combined led to the fact that Europeans had more diseases that they themselves tolerated easily. but which turned out to be fatal for the untrained Indians.
It should be noted that in Africa, where the population density was higher and cattle breeding was developed quite strongly, Europeans often suffered from local diseases to which Africans had immunity.
(answer source - book by D. Diamond "Guns, Microbes, Steel")