system
Given the following post, respond as if you're commenting on Reddit:
human
When Europeans settled America, why didn't ~90% of the settlers die from Native American diseases? Why did Europeans have the stronger immune system and not vice versa?

I just read an article that explained that the Europeans brought the diseases that killed tens of millions of the American Natives. Yet Columbus and Giovanni lived to tell of their discoveries. Surely there were deadly diseases that the Natives had, right?
gpt
The European settlers were from closely packed cities which is a good way of spreading disease. The disease came from the domesticated animals that many of the Europeans were in close contact with. The Europeans had diseases that wiped out many (Black Plague for example) but they also had a chance to build up antibodies to the diseases.

The American Natives had no previous exposure to the diseases of the Old World. That meant the diseases could strike them down without any resistance. The New World peoples did not live as closely nor have domesticated animals. 

IIRC, however, the Americas did introduce one particularly deadly disease to the Europeans. That was syphilis. Seems appropriate for some reason.