Guns, Germs and Steel (Diamond 1997) argues that Old World invaders of the New World c. 1500 carried high-mortality infections to Native Americans, but the latter couldn't similarly infect the former, because such diseases' existence results from zoonoses in cities where both humans and domesticated animals have a high population density. This is the first of two videos giving a much shorter discussion of the idea than the book-length one due to Diamond.
But there was a limited presence of Norse people in the Americas c. 1000 AD. Did this also infect natives with something deadly to which they lacked immunity? If not, why not? I've found one online discussion of why this may not have happened, but I was hoping for something more concise than 615 comments. As best I can tell, the short answer is Norse practices didn't predispose them to such infections, but I'm unclear on the details.