Cortez conquered the Aztecs starting in 1519. Two huge epidemics in the 1540's wiped out 80% of the native population. Eighty percent, over sixteen million people, died, leaving only two million. Demography is destiny.
It had been commonly assumed that one of the European illnesses, such as smallpox or typhus, was the killer, partly because most Spanish, priests excepted, were little affected by it. But it seems now that it was a hemorrhagic fever, like hanta, native to the New World. The outbreaks followed the same pattern as the 1993 hanta: several years of drought followed by wet weather.