I read that there was a disease epidemic that spread throught North and South American that eradicated large populations before the arrival of the Europeans. It seemed to mirror the time of the bubonic plague in Europe.
The theory is that Basque Fishermen, who were fishing off new England —brought the diseases before Columbus and Spanish Exploration. Its possible—there was contact before Columbus—we know of the Vikings—and their may have been many others. Oak Island imply lots of groups.