Since yesterday was Columbus Day/Indigenous People's Day, I was remembering all of the problems that Columbus caused when he landed in the Americas- some argue that up to 90% of the indigenous population was killed by disease. I was also thinking about how the Vikings had actually "discovered" the Americas first, and IIRC there was some contact with the native population.

So, why didn't the disease spreading start then? Is there evidence of disease hitting the Americas before Columbus? Thoughts on this?