system
Given the following post, respond as if you're commenting on Reddit:
human
Why did Eurasian diseases not transfer to all of North America when the Vikings made contact, instead devastating the Americas post-Columbus?

Or maybe they did but the groups they killed were themselves isolate? But that seems so unlikely to me.

I realize the answer is probably something like "there wasn't *much* contact" or even "that contact is unproven" so my larger question is: how much contact do you have to have before disease ravages a vulnerable population? How much does chance play a role, and how precise is our knowledge of this kind of thing?
gpt
So, the vikings got to North America in an expedition from Greenland, which was itself an expedition from Iceland, which was itself colonized from Scandanavia.   Epidemic diseases don't persist well in small populations, and Greenland was a small, isolated population colonized from a fairly small, fairly isolated population.  So probably no epidemic diseases managed to hop from island to island to island the few times the connection was made.  

It's not a straightforward X percent of contact.  You could have one guy wash ashore freshly dead of smallpox and that could do the trick, or you could have a fair amount of contact provided no one carrying the disease happened to get by (long travel times and intermediate steps in the chain make transmission less likely).  But the nature of the Norse contact just didn't allow anything too nasty to take the jump.