How did European diseases affect the Americas?
European diseases — especially smallpox, measles, and influenza — caused the greatest demographic catastrophe in human history, killing an estimated 90% of the indigenous American population within a century of contact. This 'Great Dying' destroyed civilizations, shattered resistance to colonization, and fundamentally altered the course of American and world history.
The biological impact of European contact on the Americas was the most devastating demographic event in recorded history. Indigenous peoples, having been isolated from Eurasian disease environments for over 10,000 years, had no immunological resistance to a suite of diseases that were endemic in the Old World. The result was catastrophic population collapse on a scale unprecedented in human experience.
Smallpox was the most lethal killer, but it was far from alone. Measles, influenza, typhus, bubonic plague, cholera, malaria, diphtheria, whooping cough, and other diseases swept through indigenous populations in successive waves. Each disease found a completely immunologically naive population. A smallpox epidemic might kill 30–50% of a community, and before survivors could recover, measles would arrive, followed by influenza. The compounding effect of multiple epidemics was devastating.
The scale of death is difficult to comprehend. The pre-contact indigenous population of the Americas is debated, but modern estimates range from 50 to 100 million people. By 1600, the population had fallen to perhaps 5–10 million — a decline of 80–95%. The Aztec Empire, with a population of roughly 20 million at contact, was reduced to about 1 million within a century. The Inca Empire experienced similar devastation. Entire peoples vanished, their languages, cultures, and knowledge lost forever.
The political and military consequences were decisive. Disease often preceded European armies, weakening or destroying indigenous societies before colonizers arrived. Cortés conquered the Aztec Empire with a few hundred soldiers partly because smallpox killed far more Aztecs than Spanish weapons did. Without the biological catastrophe, the small numbers of European colonizers could not have dominated continents populated by millions.
The labor shortage created by indigenous population collapse directly drove the Atlantic slave trade, as colonizers imported enslaved Africans to work plantations that indigenous peoples could no longer be forced to labor on. The Great Dying thus linked the fates of three continents in ways that continue to shape the modern world.