An interesting counter-factual that comes to mind - if the Norse Greenlanders had brought smallpox or other diseases with them, then Native Americans would have had 500 years to recover (and keep immunity?) - The conquistadors would have faced millions of not-dying-natives. A much different world would have resulted.
That's really interesting! Of course, if the native population died off around them, the Vikings would probably have expanded their settlements and perhaps ruled North America instead.
But then again farming populations can expand really fast with low population pressure. The 13 colonies were seeing their populations double every generation just from natural fertility even before immigration.