Rats have long been blamed for spreading the parasites that transmitted plague throughout medieval Europe and Asia, killing millions of people. Now, a provocative new study has modeled these long-ago outbreaks and suggests that the maligned rodents may not be the culprits after all.

The study, published on Monday in the journal PNAS, instead points the finger at human parasites—such as fleas and body lice—for primarily spreading plague bacteria during the Second Pandemic, a series of devastating outbreaks that spanned from the 1300s to the early 1800s.

These outbreaks include the infamous Black Death, which wiped out a third of Europe’s population in the mid-1300s, amassing a body count in the tens of millions.

“The plague really transformed human history, so