Not sure why the downvotes? The number was really that high and this is the basic reason the Aztecs were defeated:

https://www.theguardian.com/world/2018/jan/16/mexico-500-yea...

Within five years as many as 15 million people – an estimated 80% of the population – were wiped out in an epidemic the locals named “cocoliztli”. The word means pestilence in the Aztec Nahuatl language.