It has long been thought that the plague, caused by the bacterium Yersinia pestis, which lasted in Europe until the early 19th century, was spread by rats. But now scientists from the University of Oslo and the University of Ferrara believe human “ectoparasites”, such as body lice and human fleas, might be more likely to have caused the epidemic.

Using mortality data from nine plague outbreaks in Europe between the 14th and 19th centuries, the teams tracked how the plague developed.