What do you mean by massive?
Massive = 90%+ population reductions in less than a century.
A previous poster lists a single epidemic reducing population by 50%, similar to that of the Black Death in Europe.
The difference is that the native Americans weren’t hit by a single plague, but by repeated epidemics of different diseases. 50% here, and then 25% of the survivors a few years later, and then 20% of the survivors a few years after that, and you’re approaching 90% pretty quickly.
What happened to the original Americans isn’t hard to understand. The Plague of Athens, in Pericles’ time, the Plague of Justinian, the Black Death, smallpox and multiple others had hit the Old World centuries apart. Populations, while often reduced by 25 to 50%, had time to recover and adjust between epidemics.
The New World got hit by essentially all these diseases in less than a century. Entire civilizations and peoples disappeared long before a single European contacted them. Those who survived went through a spiritual crisis unrivaled in history. Just look at the psychological impacts of the Black Death in Europe and magnify it by a couple orders of magnitude.
Many of the “primitive” peoples the Europeans ran into may have been no such thing. They were the traumatized survivors of the demographic equivalent of a nuclear war.