For the past hundreds of years, flea-infested and dirty rats have taken the blame for the spread of the bubonic place throughout medieval Europe, which killed millions of people.

However, a new study published in the journal PNAS on Jan. 15 suggests that the maligned rodents were not the culprits. Instead, the blame can be put on human parasites like body lice and fleas for the primary spread of the plague bacteria during the Second Pandemic.

It was a deadly time that included episodes of terrible outbreaks from the 14th century to the early 19th century. The outbreaks, which included the devastating Black Death, annihilated one-third of the population in Europe during the mid-1300s.

Human Parasites Infected With The Yersinia Pestis Bacteria

Scientists have long held the notion that human fleas and lice spread the Black Death. That's because they picked up the plague by biting an infected person, then, they could have hopped onto another human and infected them with the disease.

When fleas infected with the Yersinia pestis bacterium bite people, the bacteria can get into the bloodstream and congregate in the lymph nodes of the infected person. Lymph nodes are present throughout the body and the infection can cause them to swell into the horrible buboes, which gave the bubonic plague its name.

Model-Based Research Show That Human Parasites Caused The Medieval Plague

To test if the plague spread through rat-fleas or human-parasites, the research team modeled each with equations that mimicked an outbreak’s rise and fall, based on how body lice, fleas, or rats would behave and spread the disease.

“It’s basically bookkeeping—you see how people move [in the simulation],” study coauthor Boris Valentijn Schmid said.

The research team ran their models numerous times to evaluate which of them were perfect fits for the mortality patterns from the Second Pandemic’s nine different European outbreaks of plague. The scientists discovered that in seven of the nine cities they had analyzed, the human-parasite model was better matched to the mortality records in comparison to the rat-flea one.

“It’s a really cool piece of work,” said system scientist Charles “Chick” Macal, who was, however, not involved with the study. “It gets at the underlying question of why these outbreaks occur at all.”

For now, the research team has said that more experimental data is needed to improve their models. They also agree that study is going to rake up controversy among plague scientists, especially those who are insistent that rats were the major cause of the medieval plague outbreaks.