ghug,
It depends what you mean. A very large number of Native Americans did die from disease, both immediately before Europeans arrived and, in larger numbers, after Europeans arrived, because they had no immunity to European disease. Tragic as this was, it is obviously absurd to call it any kind of genocide or murder.
(An exception would be if some of it was deliberate. It is controversial whether the Spanish did engage in some deliberate smallpox poisoning. There is no direct evidence that they did).
As for actual numbers killed, though, whether by displacement or direct violence or otherwise, it was certainly far, far short of 70 million. We have relatively poor records in the deep past, of course, but consider that the entire US population in the 1780s was about 3 million, and ask yourself just how they might have managed to kill an order of magnitude more people than themselves (yes, they had guns, but no, the massacres, terrible as they were, did not reach that level).
As history progresses, we do have progressively better records, and there is nothing in the 19th century to support getting to those numbers after starting so far from them at its beginning.