system
Given the following post, respond as if you're commenting on Reddit:
human
Why didn't the Vikings unleash apocalyptic plagues in the new world centuries before Columbus?

So it's pretty generally accepted that the arrival of Columbus and subsequent European expeditions at the Caribbean fringes of North America in the late 15th and early 16th centuries brought smallpox and other diseases for which the natives of the new world were woefully unprepared. From that touchpoint, a shock wave of epidemics spread throughout the continent, devastating native populations, with the European settlers moving in behind it and taking over the land.

It's also becoming more widely accepted that the Norse made contact with the fringes of North America starting around the 10th century and continuing for quite some time, including at least short-term settlements if not permanent ones. They clearly had contact with the natives as well.

So why the Spaniards' germs and not the Norse ones?
gpt
To understand this you need to understand the nature of epidemic diseases and the Viking voyages of exploration (as opposed to the later ones of Columbus).  

Epidemic diseases in general do not persist well in small isolated populations.  They tend to spread rapidly, making everyone immune or dead.  

The Vikings did not sail directly from Norway to North America.  Their ships probably weren't up to the task of making the crossing all at once, at least not reliably. Instead, they colonized Iceland, and a small group colonized Greenland, and a subgroup of that group went to North America.  The population living on Iceland was fairly small, and the number living on Greenland was very small.  As a result, it would have been quite difficult for a disease to make it all the way across.  Some ship would have had to carry the disease to Iceland, where it would have had to persist in the population long enough for someone infected to get around to sailing to Greenland (and not die on the way), where it would have had to persist in that population long enough for someone to sail over to North America, where some unlucky native would have had to catch it and spread it from his tribe off of Newfoundland and out into the rest of the continent.   That's a lot of low probability events, especially since ships did not pass all that frequently to Greenland or even at times Iceland.  Contrast this with Columbus et. al.  leaving from populated, disease-ridden cities in Europe and sailing right over to the Americas.  All you need in that case is a sick sailor to make the passing.