The rodents have long been blamed for spreading the plague, which is caused by the bacterium Yersinia pestis. Researchers thought the bacterium would infect fleas on rats, and when the rats died, the fleas would jump to humans, infecting them. Rats are also thought to be responsible for spreading more recent cases of plague, including the outbreak in Madagascar in 2017.
But the long-held theory that rats spread the plague during the Second Pandemic in the 14th to 19th centuries had a problem—the disease did not spread quickly enough. This discrepancy spurred Katharine Dean, an ecologist at the University of Oslo, and her colleagues to study the patterns of how disease moves through a population. “The plague really transformed human history, so it’s really important to understand how it was spreading and why it was spreading so fast,” Dean tells National Geographic.
The researchers simulated the outbreak of the plague in nine European cities between 1348 and 1813. They modeled the spread of the disease by human fleas and lice, rat fleas, or transmission through the air. In seven of the nine cases, human fleas and lice best matched the pattern of outbreak, compared to the other two modes of transmission.
“The conclusion was very clear. . . . The lice model fits best,” study coauthor Nils Stenseth, a biologist at the University of Oslo, tells the BBC. “It would be unlikely to spread as fast as it did if it was transmitted by rats.”
The takeaway, Stenseth says: personal hygiene. The disease doesn’t spread if we stay clean. Another tip, he tells the BBC, “If you're ill, you shouldn't come into contact with too many people. So if you're sick, stay at home.”