People are weirdly against this idea, I think because they believe it diminishes the deliberate genocide that came later, which it doesn't. The horrible truth is that disease spread through completely biologically defenseless populations starting in the late 15th century. By the time European countries were consolidating colonial power, the Native population had been obliterated by somewhere between 65–89%. Those aren't extremes, that's a range of completely plausible figures. The variance is so large because it's hard to tell how many people used to live in a place when disease, unaided, killed every person in every settlement in unthinkably huge areas. To say entire tribes disappeared is an understatement, entire networks of multiple cultures were wiped out so thoroughly that their memory is lost forever. The Native American population in 1800 was a small fraction of the number of people who once lived.