It seems a bit aggressive to attribute the disappearance to European diseases. Is there any evidence to support this?
In general, European-borne diseases wiped out millions of indigenous populations in North, Central and South America. That's well documented. In this articles and others, I think scientists speculate that since the diseases were so wide-spread, easily spread, and did such damage in documented circumstances, that it's not a stretch to conclude that similar may have occurred in other undocumented societies from a similar timeframe.