Smallpox Devastates Tenochtitlan and the Population of the Americas
A disease unknown in the Western Hemisphere before 1520 kills as much as half the population of the Aztec capital and tens of millions across the continent
Quick facts
- Pathogen
- Variola virus (smallpox)
- Location
- Tenochtitlan and greater Mesoamerica
- Duration
- At least 70 days, beginning September 1520
- Estimated toll
- 5 to 8 million dead across Mesoamerica in the initial outbreak
- Primary source
- The Florentine Codex, compiled from Nahua eyewitness testimony
What happened
Smallpox reached the Valley of Mexico in September 1520 during the fighting between Hernan Cortes's forces and the Aztec Empire, and the outbreak lasted at least seventy days. Residents of Tenochtitlan identified the disease in Nahuatl as hueyzahuatl, great leprosy, and totomonaliztli, pustules; survivors quoted in the Florentine Codex described being covered in agonizing sores from head to foot, unable to move their limbs, and many who could not forage for food died of starvation rather than the disease itself because caregivers were incapacitated too. Current estimates put deaths at five to eight million across Mesoamerica in this single outbreak, with mortality rates in unexposed populations ranging from 30 to 100 percent of those infected. It was the first of repeated epidemics, smallpox followed by measles, typhus, and other Old World diseases, that struck Indigenous populations across the Americas in the following century.
Why it matters
Indigenous peoples in the Americas had no prior exposure to smallpox, measles, or influenza, all diseases that evolved among Old World populations living in dense contact with domesticated livestock, so these outbreaks were true virgin-soil epidemics with no partial immunity to slow them. The resulting population collapse, with Central Mexico's population falling from roughly 25 million in 1519 to about 1 million by 1600 according to demographic estimates, crippled the logistical and military capacity of Indigenous states at the exact moment European colonization was expanding, a connection historians treat as central to how the conquest succeeded.
How we know
The Florentine Codex, compiled by the Franciscan friar Bernardino de Sahagun from testimony of Nahua survivors and scribes in the decades after the conquest, is the primary Indigenous-language eyewitness source. Dumbarton Oaks, the Harvard-affiliated research institute for pre-Columbian studies, maintains a documented online exhibit synthesizing this and other period evidence.
Sources
- Dumbarton Oaks. The Great Epidemic of 1520 · Reputable sourcedoaks.org · The domain "doaks.org" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
- Dumbarton Oaks. The Great Epidemic of 1520 · Reputable sourcedoaks.org · The domain "doaks.org" is on our Reputable source registry. · Link is live and its text matches the event's key terms (Jul 2026)
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Related timelines
- The Age of Exploration → · See how Old World contact and colonization drove the demographic collapse across the Americas alongside conquest by arms.