In 1665 and 1666, one city experienced two enormous tragedies: the Great Plague of London and the Great Fire of London. The plague killed roughly 15 to 20 percent of the city’s population, while the fire burned about a quarter of London’s metropolis, making around 100,000 people homeless. And though the city only officially recorded a small number of deaths from the fire, the real death toll was likely quite high.

Humans often want to find a silver lining amid disaster, and one myth that sprung up around these twin tragedies is that the Great Fire ended the Great Plague by driving out the rats who were spreading the disease.

“I was raised with that myth,” says Adrian Tinniswood, a senior research fellow in history at the University of Buckingham, England, and author of By Permission of Heaven: The Story of the Great Fire. “It was the kind of standard school talk back in the ‘60s when I was growing up.”

The Great Plague was London’s last major outbreak of the plague, a bacterial infection caused by Yersinia pestis. The outbreak began in the late winter or early spring of 1665. By the time King Charles II fled the city in July, the plague was killing about a thousand people a week. The death rate peaked in September when 7,165 people died in one week.

Officially, the city recorded 68,596 deaths from the Great Plague, and the true death toll may have exceeded 100,000. Most of these deaths were from bubonic plague, a form of plague spread through fleas on small mammals. In London, the major carriers were rats. (In the United States, where plague has likely existed since a 1900 outbreak in San Francisco, squirrels and prairie dogs can and do transmit plague to humans.)

After peaking in September 1665, the city’s plague deaths began to taper off that winter. In February 1666, King Charles II returned to London, signaling a belief that the city had become “reasonably safe,” says Christoph Heyl, the chair of British literature and culture at the University of Duisburg-Essen, Germany, who has written about the Great Fire.

So although London continued to report plague victims until 1679, the major outbreak was mostly over by September 2, 1666, the night a baker named Thomas Farriner unwittingly started the Great Fire of London.

“In a world lit only by fire, people were always setting fire to their houses,” Tinniswood says of England in the mid-1600s. What was more unusual was for someone to start a fire in his house that set over 13,000 other houses on fire—which is what Farriner accidentally did.

The Great Fire destroyed most of the official city of London (which was geographically smaller than modern-day London), but it didn’t reach many of the outer metropolitan areas like Whitechapel, Clerkenwell and Southwark that were also affected by the plague. This means that even if the fire did drive out rats in the 436 acres it burned, it didn’t spread far enough to drive out all of the plague-spreading rats in greater London.

In fact, data suggests the fire didn’t have any effect on the plague. Plague deaths in London were already declining by the time the fire started, and people also continued to die of the plague after the fire. It’s not clear when exactly people began to say that the fire ended the plague, since people didn’t seem to believe this at the time.

“If you look at the discourse of the time, there was never any connection between the end of plague and Great Fire,” Heyl says. As an example, he points to a piece of royalist propaganda that tried to spin the Great Fire as a kind of political success for King Charles II. “Even in a text like this, there’s no trace of a connection whatsoever between fire and the end of the plague.”

The uncomfortable fact is historians don’t really know why the Great Plague ended. After the fire, London strengthened old building codes that favored brick over timber because it’s less flammable. Brick is also harder for rats to burrow into, but as Museum of London curator Meriel Jeater notes, there were no concurrent hygienic or sanitary improvements with this brick use that might have explained an eradication of plague.

Even in the 21st century, the plague remains a serious disease. Between August and November 2017, a plague outbreak in Madagascar led to 2,417 infections and 209 deaths. Antibiotic treatment is extremely effective against the plague. But when the disease isn’t diagnosed or antibiotics aren’t available, it can still be very fatal, just as it was back in 1665 and 1666.