Also known as Columbus Day.
1491, a really good book, makes an awful lot of the spread of diseases introduced by Europeans and how this weakened native societies so that Europeans were able to conquer the Americas much easier than they otherwise would have. But there's a problem. The problem is with the notion that diseases were the only factor. I'm not saying that 1491 is saying this--it's more nuanced than that--but that the diseases plus the notion of less population in North America idea is insufficient to explain why there are not that many Native Americans left in the United States.
1491 points out that Native Americans in both North and South America are much closer genetically to each other than populations in the old world occupying similar areas of land. You would think, then, that there would be similar responses to disease. Yet for some reason we have nations like Ecuador, Bolivia, and Peru that are majority indigenous while the contains very few Native Americans in relation to the rest of the population. There are also native people's in Mexico--beyond Chiapas--who preserve their identities and their language, or at least try to, while also participating in Mexican culture to one extent or another.
None of these populations of people is uncontacted, to say the least. Diseases have been through all of them. Yet indigenous communities thrive on the one hand and are pushed into deserted areas and marginalized in the other. There has to be some other factor to account for the differences in population of Native Peoples between the United States and Latin America.
That factor is probably the policy towards Native Americans started by the British and augmented by the newly independent U.S., that featured an all or nothing approach to Native Peoples---either eliminate them or leave them alone but not let them into the new society as fully participating members. Human action and human choices are the reason why Native Americans have such a limited presence in the United States--not disease susceptibility.