Disease and the Collapse of Amerindian Populations

Disease ran one direction across the Atlantic because of what each hemisphere had to catch germs from. Eurasians and Africans had lived in close, dense contact with domesticated cattle, pigs, horses, and fowl for thousands of years. Herd animals pass pathogens to humans, and over generations the people who survived those crossover infections built up immunity, while the diseases themselves evolved into human-to-human killers like smallpox and measles. The Americas had very few domesticable large animals, so Amerindian populations had never been exposed to this whole category of disease and carried no inherited or acquired defenses. When contact began, the exchange of germs was wildly lopsided.

The results moved faster than the Spanish and Portuguese themselves. Smallpox reached central Mexico in 1520, a year before Tenochtitlan fell, likely carried by an infected member of a relief expedition, and it killed Aztec emperor Cuitlahuac along with huge numbers of soldiers and civilians. Measles and influenza followed in later waves, and because American populations had no immunity, each epidemic could kill a third or more of a region in a single year. Disease routinely outran any European column, moving through trade and travel networks and striking communities that had never seen a European.

This collapse explains why tiny Spanish forces could topple the Aztec and Inca empires: they were fighting states already gutted by death, succession crises, and collapsed food and labor systems, not simply outfighting intact civilizations. It also set up the next piece of Unit 4. As Amerindian labor died off by the millions, Spanish and Portuguese colonizers turned increasingly to forced labor from West Africa, making the demographic catastrophe in the Americas a direct cause of the growing Atlantic slave trade.