Did a deadly salmonella strain cause the mysterious 16th-century Aztec cocoliztli epidemic?

History · Published 3 October 2026 Swipe more like this →

In short: Between 1545 and 1550, a catastrophic outbreak known as cocoliztli killed up to 15 million people in Mexico. For five centuries, the cause remained a mystery. Now, ancient DNA extracted from teeth in a mass burial site suggests a specific, lethal strain of salmonella may be to blame.

The cocoliztli epidemic, which decimated indigenous populations in 16th-century Mexico, has long baffled historians. Contemporary accounts described victims suffering from high fevers, nosebleeds, and coughing up blood. Because these symptoms do not leave visible marks on skeletal remains, researchers turned to ancient DNA analysis. By testing samples from 24 individuals buried at the Teposcolula-Yucundaa site, scientists identified Salmonella enterica, specifically the Paratyphi C strain, in 10 of the remains.

This strain causes an enteric fever similar to typhoid and is transmitted through contaminated food or water. While the bacteria was found in the remains of a Norwegian woman who died in 1200, it was absent in five individuals buried before European contact. This has led some researchers to hypothesize that the pathogen was introduced to the Americas by Europeans, who brought various infectious diseases to which indigenous populations lacked immunity.

However, the role of salmonella remains contested. Experts suggest that multiple factors likely converged to create the disaster, including the severe social disruptions of colonialism, famine, and forced relocations. Furthermore, the outbreak occurred during a period of intense megadrought, which likely compromised sanitation and hygiene. Some scholars argue that while salmonella was present, other agents—such as viral hemorrhagic fevers carried by rodents—may have also played a significant role in the massive death toll.

Confirming the exact cause of cocoliztli remains a challenge, as researchers currently have limited DNA samples from only one burial site. Until more archaeological evidence is uncovered, the scientific community views the salmonella finding as a compelling piece of a much larger, complex puzzle involving environmental stress, social upheaval, and the introduction of new pathogens to a vulnerable population.

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