Epidemics and the Collapse of Native Populations

Europeans, Africans, and Asians had lived alongside smallpox, measles, and influenza for thousands of years. Generation after generation, those diseases killed the most vulnerable and left survivors with immunity, and eventually some genetic resistance spread too. The peoples of the Americas had been isolated from Afro-Eurasia for over ten thousand years and had never met these pathogens. When smallpox arrived with Spanish ships, it hit a virgin-soil population: everyone was vulnerable at once, not just infants. Entire villages fell sick in the same week, with no healthy adults left to farm, haul water, or care for the dying. That alone could kill thirty to fifty percent of a community in a single outbreak, and waves of measles, typhus, and influenza followed over the next century.

Central Mexico shows the scale. Around 1520, before contact with Cortes's expedition, the region held an estimated 22 million people. A smallpox epidemic broke out in 1520 while Cortes was fighting the Aztecs, killing the emperor Cuitlahuac and devastating Tenochtitlan's defenders. Further epidemics followed in 1545 and 1576. By 1620, the population had fallen to roughly 1 million, a loss of over ninety-five percent in a century. Spanish steel, horses, and gunpowder helped win battles, but armies of a few hundred could never have defeated a city of hundreds of thousands through weaponry alone. What tipped the balance was disease weakening Aztec leadership and manpower, combined with tens of thousands of Indigenous allies who joined Cortes against Aztec rule.

This collapse sets up everything that follows in the unit: Europeans reading mass death as divine favor or Native inferiority, labor shortages that pushed colonizers toward African slavery, and the broader Columbian Exchange reshaping ecosystems and societies on every continent it touched.