From 1545 to 1550, Aztecs in what is today southern Mexico experienced a deadly outbreak. Anywhere from five to 15 million people died. Locally, it was known as cocoliztli, but the exact cause or causes has been a mystery for the past 500 years.

Now, a new study published in the journal Nature Ecology and Evolution suggests the outbreak could have been caused by a deadly form of salmonella.

According to study author Åshild Vågene from the Max Planck Institute, the strain is a bacterial infection that causes a type of enteric fever nearly identical to typhoid. While that specific strain of salmonella is much rarer today, Vågene says it would have spread similarly. Any food or water