Cocoliztli Epidemic

The cocoliztli epidemic of 1545 to 1548 was one of the deadliest outbreaks in the history of the Americas. A violent hemorrhagic fever swept through central Mexico, then called New Spain, and killed a very large share of the Indigenous population. Estimates of the toll run from several million to perhaps 15 million deaths, though these figures are uncertain and debated. The epidemic matters because it deepened a demographic collapse that was already under way and reshaped colonial society, labor and land ownership.

The background was the Spanish conquest of the Aztec Empire in 1521. In the decades that followed, Indigenous communities faced smallpox, measles and other imported diseases, along with forced labor, tribute demands, displacement and disrupted farming. These pressures weakened populations before 1545. Severe drought is also thought to have struck the region in the years before and during the outbreak, which may have increased hunger and crowding around water sources and made disease easier to spread.

"Cocoliztli" is a Nahuatl word usually translated as "pestilence." Spanish and Indigenous observers described a terrifying illness marked by high fever, headache, bleeding from the nose, eyes and mouth, jaundice, and severe thirst, with death often coming within days. Physicians such as Francisco Hernández later documented similar symptoms in a second epidemic in the 1570s. In 1545 the disease spread through highland communities, and Nahua writers recorded the devastation. Whole towns were left with too few healthy people to bury the dead or tend crops. Indigenous people were hit much harder than Spaniards, though the exact reasons are unclear.

The cause is still disputed. For a long time many historians suspected a native hemorrhagic virus, possibly carried by rodents, or an imported disease such as typhus or smallpox in an unusual form. In 2018, researchers analyzing DNA from skeletons in a cemetery at Teposcolula-Yucundaa in Oaxaca reported finding Salmonella enterica Paratyphi C, which causes enteric fever. This suggests that a European-introduced bacterium may have played a role, though some scholars remain cautious. Evidence for a single cause is not settled, and more than one pathogen may have been involved.

The consequences were lasting. Population losses undermined Indigenous communities and made it harder to meet colonial labor and tribute demands, which in turn led to harsher pressure on survivors. Spanish authorities responded with measures such as consolidating scattered villages into larger settlements, which often made the spread of disease easier. Land left vacant by the dead was claimed by Spanish settlers, helping the growth of large estates. The Spanish crown's reliance on enslaved Africans also grew in some regions as Indigenous labor shrank. Cocoliztli returned in 1576 to 1581 with similar horror, and the two epidemics remain central to understanding how disease and colonialism combined to transform the Americas.