In 1545, an unknown disease struck the Aztec Empire. Those who came down with it might become feverish, start vomiting, and develop blotches on their skin. Most horrific of all, they’d bleed from their eyes, mouth, and nose, then die within a few days.

Over the next five years, the disease—then called “cocoliztli,” or “pestilence”—killed between seven and 17 million people. Scientists and historians have long wondered what the source of this mysterious epidemic was. Now, a group of researchers may have found the answer: salmonella.

On January 15, 2017, the scientific journal Nature Ecology & Evolution published a study of Salmonella enterica bacteria in the teeth of cocoliztli victims. Most Americans know salmonella as a foodborne illness that you can get if you eat, for example, raw eggs or chicken.

Though S. enterica was the only germ that researchers detected in the victims’ teeth, they do caution that other indetectable pathogens could have been involved, too.

“We cannot say with certainty that S. enterica was the cause of the cocoliztli epidemic,” Kirsten Bos, a molecular paleopathologist at the Max Planck Institute in Germany and co-author of the recent study, told The Guardian. “We do believe that it should be considered a strong candidate.”

European invaders brought many new and devastating illnesses to the Americas in the 16th and 17th centuries. It’s possible that Spanish invaders brought salmonella to the Aztecs in modern-day Mexico through domesticated animals.

The study doesn’t pinpoint the source of the bacteria, leaving open the possibility that it originated in the Americas. Yet even if the Spanish didn’t bring the bacteria, they likely still played a role in how it affected the Aztec people.

“We know that Europeans very much changed the landscape once they entered the new world,” Bos told NPR. “They introduced new livestock, [and] there was lots of social disruption among the indigenous population which would have increased their susceptibility to infectious disease.”