Did mice spread the plague?

Rats might not be responsible for medieval plague outbreaks after all

"Basically it's accounting - you can see how people move [in the simulation]," says co-author Boris Valentijn Schmid, a biologist from the University of Oslo.

After Dean and Schmid had run through the models many times, they statistically evaluated which models most closely matched the patterns of nine different European plague outbreaks after 1300. To their surprise, they found that in seven out of nine cities studied, the model for human parasites matched the death rate records better than the model for rat fleas.

"That's pretty cool work," says Charles Macal, a scientist at Argonne National Laboratory who models disease spread but was not involved in this study. "It addresses the underlying questions of why these outbreaks occurred in the first place."

According to Dean and Schmid, there is still room to improve their models with further experimental data. They also acknowledge that their study is likely to spark controversy among scholars studying the plague. Some of them are downright passionate about the belief that the medieval outbreaks were caused by rats.

"There are a lot of hot debates about the plague," says Dean, who regards herself and Schmid as objective observers in this case. "We don't have a horse in the race."

The article was originally published in English on NationalGeographic.com.