Wouldn’t any longer term settlers in the Americas have brought their diseases with them? The way small pox ran through the Native American community after the Spanish arrived you might imagine exposure 1,000 years earlier would have inoculated the population.
When the Pilgrams came to Mass Bay, they found abandoned Indian villages. Then a young Indian helped them learn the classic corn, beans, squash agriculture with a fish in the hole for fertilizer. Saved their lives, they found that many Indians had died from disease, probably smallpox from Vikings.