Rats have long been blamed for spreading the Black Death around Europe in the 14th century. Specifically, historians have speculated that the fleas on rats are responsible for the estimated 25 million plague deaths between 1347 and 1351.

However, a new study suggests that rats weren’t the main carriers of fleas and lice that spread the plague—it was humans.

In a study published in January 2017 in the Proceedings of the National Academy of Sciences, researchers simulated Black Death outbreaks in European cities to try and understand how the plague was spread. In their simulations, they looked at three possible models for infection: rats, airborne transmission, and fleas and ticks that humans carry around with them on their bodies and clothes.

A flea infected with the plague. (Credit: Smith Collection/Gado/Getty Images)
A flea infected with the plague. (Credit: Smith Collection/Gado/Getty Images)

In most of the cities, the model that focused on fleas and ticks on humans was the most accurate model for explaining the spread of the disease.Though it may come as a surprise to most readers, previous studies have backed up these findings. The consensus seems to be that the plague spread too fast for rats to be the culprit carriers.  

“It would be unlikely to spread as fast as it did if it was transmitted by rats,” Nils Stenseth, a professor at the University of Oslo and co-author of Monday’s study, told BBC News. “It would have to go through this extra loop of the rats, rather than being spread from person to person.”

It’s not clear where the belief that rats spread the plague came from in the first place. After all, the researchers write that “there is little historical and archaeological support for such a claim.” For example, if rats really were a main cause of the plague, there would be more archaeological evidence of dead rats.