The Cocoliztli Epidemic or the Great Pestilence was an outbreak of a mysterious illness characterized by high fevers and bleeding which caused 5–15 million deaths in the North American territory of New Spain during the 16th century. The Aztec people called it cocoliztli, Nahuatl for pestilence. It ravaged the Mexican highlands in epidemic proportions, resulting in the demographic collapse of some Indigenous populations.
Based on the death toll, this outbreak is often referred to as the worst epidemic in the history of Mexico. Subsequent outbreaks continued to baffle both Spanish and native physicians, with little consensus among modern researchers on the pathogenesis. However, recent bacterial genomic studies have suggested that Salmonella, specifically a serotype of Salmonella enterica known as Paratyphi C, was at least partially responsible for this initial outbreak. Others believe cocoliztli was caused by an indigenous viral hemorrhagic fever, perhaps exacerbated by the worst droughts to affect that region in 500 years and poor living conditions for Indigenous peoples of Mexico following the Spanish conquest (c. 1519).
Why this event still matters
Historical incidents shape the rules that protect people today — building codes, evacuation policy, warning systems and inspection regimes almost always trace back to a specific disaster. Studying cocoliztli epidemic alongside current events is how preparedness improves.
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