As soon as the Spanish made prolonged contact with appreciable numbers, the whole Eurasian disease complex was unleashed on the Indians. 90 percent mortality is the conventional figure that flies around. The disease wave, of course, hit the tribes mostly before the first direct contact with white explorers. This is why North America seemed so empty to the newcomers, who did not realize they were moving across a vast killing field, slowly reverting to nature.
The Indians may have returned the favor by giving syphilis to the Europeans, although that is disputed. There is no doubt that the exchange of diseases was asymmetrical.
The earlier Viking settlements must have been too small and too short-lived, with a too healthy population, to have had a similar effect. Had the Vikings held on for a longer time, and had their settlements grown, the Indian die off would have occurred centuries earlier. The only hope for the Indians would have been to postpone first contact for another 400 years, after the development of the germ theory of disease.
Well, the “good” news is that those from both continents alive today are from the survivors. i.e. they are genetically “stronger”.