The Catastrophic Demographic Collapse in 16th-Century Central Mexico

Primary Cause: Eurasian Pathogens

The deaths of an estimated 20 million people (80–90% of the pre-contact population) in central Mexico during the 1500s were overwhelmingly caused by Old World infectious diseases—especially smallpox, measles, influenza, typhus, and mumps—introduced by Spanish conquistadors and subsequent colonists.

Why So Devastating?

• No prior exposure: Indigenous immune systems had zero memory of these pathogens.
• Simultaneous epidemics: Waves hit every 10–15 years (1520 smallpox, 1531 measles, 1545 cocoliztli—possibly salmonella, 1576 cocoliztli).
• Social disruption: Famine, forced labour, displacement, and warfare amplified mortality.
• Genetic homogeneity: Limited HLA diversity reduced population-level resistance.

Beyond Disease

Violence, enslavement, cultural destruction, and ecological change (livestock overgrazing, deforestation) compounded the collapse. But the epidemiological shock was the overwhelming driver—one of the greatest demographic disasters in human history.