Europeans did bring back disease they weren't immune to:

Europeans brought deadly viruses and bacteria, such as smallpox, measles, urope. Europeans brought deadly viruses and bacteria, such as smallpox, measles, typhus, and cholera, for which Native Americans had no immunity (Denevan, 1976). yphus, and cholera, for which Native Americans had no immunity (Denevan, 1976). On their return home, European sailors brought syphilis to Europe. Although less n their return home, European sailors brought syphilis to Europe. Although less deadly, the disease was known to have caused great social disruption throughout eadly, the disease was known to have caused great social disruption throughout the Old World (Sherman, 2007)

https://www.kellogg.northwestern.edu/faculty/qian/resources/... (Note that I haven't read this whole thing, but wanted to find a source for my little factoid)

It is one of the major points from Guns, Germs and Steel. Most of our major diseases have transferred from domestic animals. For example both measles and smallpox (the two most deadly diseases for Native Americans) come from cows.

Europeans successfully domesticated animals, Native Americans did not. So Europeans had diseases that Native Americans were not adapted to.

I believe the theory is that Europeans kept various domesticated animals and Native Americans had far fewer, and that those animals were frequently sources of new diseases. Europe was also more interconnected, so diseases may have spread farther while Native diseases died out.

See the wiki page: https://en.m.wikipedia.org/wiki/Native_American_disease_and_...

ohh that makes sense, thanks