Smallpox and the fall of Tenochtitlan

Hernán Cortés landed in 1519 with 11 ships, about 100 sailors and some 500 soldiers, and by the next year he held the Aztec ruler Montezuma II prisoner in Tenochtitlan. In June 1520 Montezuma's brother Cuitláhuac succeeded him, and on the night of 30 June, the noche triste, the Aztecs drove the Spaniards from the city with heavy losses.

Smallpox arrived that year. The Franciscan friar Toribio, better known by his Mexican name Motolinía, wrote that it came in 1520 with an enslaved African in the company of Narváez and carried off half the inhabitants of Mexico; Alexander von Humboldt repeated his account in the early 19th century. The Aztecs had no immunity to the disease. Cuitláhuac died of smallpox in October 1520, after a reign of about four months, and his nephew Cuauhtémoc succeeded him.

The epidemic crippled resistance. Britannica's editors estimate that it killed about half of Tenochtitlan's people. In May 1521 Cortés attacked the city with more than 800 Spaniards and tens of thousands of Indigenous warriors, many of them Tlaxcalans, old rivals of the Aztecs, and on 13 August he took it.

How many died across Mexico cannot be known with precision. A 2002 paper in Emerging Infectious Diseases, drawing on the population estimates of Cook and Simpson, put the smallpox deaths of 1519 to 1520 at 5 million to 8 million. Later epidemics called cocoliztli, beginning in 1545 and 1576, killed millions more; the authors argued that these were probably native hemorrhagic fevers, spread by rodents and worsened by extreme drought, and not diseases brought from Europe or Africa.