The genocide didn't happen solely after the first contact, the massacre of natives lasted centuries. Many nations were wiped out in the XIX century.

And a quote for you

Proponents of the default position emphasize attrition by disease despite other causes equally deadly, if not more so. In doing so they refuse to accept that the colonization of America was genocidal by plan, not simply the tragic fate of populations lacking immunity to disease.

Professor Roxanne Dunbar-Ortiz