Peaking in the middle of the 1300s, the Black Death is widely regarded as one of the most devastating events in human history, claiming tens of millions of lives across the world.

It was caused by the Yersinia pestis bacterium and transmitted to humans via fleas, leading to plague, a disease that can be fatal within a day in its most severe forms.

There’s a lot we still don’t know about how it kicked off and spread. One open question is whether the bacterium remained in Europe since the first plague pandemic, which began with the Plague of Justinian in 541 CE, or whether it was reintroduced from outside the continent.