On the contrary I don't think the 90% death figure would make a good case for the Aztecs having a 'Great society's. The worst plague I know of was the black death and that killed about a third of the people in northern Europe. Their medical understanding and their sanitation was horrible...but at least they figured out how to do simple things like quarantine. If Mexicans managed to lose 90% of their people to a plague then they must be very stupid.