A nine-year study of cougars in the Yellowstone National Park has found that nearly half of the big cats they tracked were infected with the plague-carrying bacteria Yersinia pestis at some point, according to a paper published last month in Environmental Conservation.

The Y. pestis bacteria is behind the Black Death, the mid-1300s epidemic of bubonic plague that in five years killed over 20 million people in Europe. These days, only about seven people catch Y. pestis each year in the United States. The bacteria lives in the soil, gets picked up by fleas living on rodents, and infects other creatures on its way up the food chain. The new evidence in cougars, also known as pumas and mountain lions, shows how flexible and dangerous the pathogen is in different hosts.

The study was conducted on cougars in the southern Greater Yellowstone Ecosystem, specifically in Jackson Hole, the valley east of the Grand Teton mountain range and south of Yellowstone National Park. “You start to get a clear picture of how hard it is to be a mountain lion in Jackson Hole,” biologist and co-author Howard Quiqley tells Mike Koshmrl of Wyoming News. “If you get to be an adult mountain lion in Jackson Hole, you’re a survivor.”

The researchers found the first feline victims of plague in the middle of winter in 2006. A cougar’s tracking collar sent an alert that the animal hadn’t moved in eight hours. The researchers found the big cat, called F018, dead at the base of a large tree, with her three-month-old kitten also dead beside her.

“Everyone assumed it was starvation,” cougar program director of the wildcat conservation organization Panthera Mark Elbroch, tells Jason Bittel at National Geographic. But analysis of tissue samples revealed the true cause of death. Elbroch adds, “We were as surprised as anyone to learn that the cats had died of plague.”

Over nearly a decade, between 2005 and 2014, the researchers checked 28 cougars for signs of Y. pestis. Eleven of the cats were found after they’d died, and four of those, including the two found in 2006, died of the plague. The researchers drew blood from 17 other cougars and analyzed the samples for antibodies, the chemical footprint left behind after the immune system fights off an infection. Eight of the 17 tests came back positive.

In all, about 43 percent of the cougars studied faced Y. pestis infections.

“The most valuable data here are the repeated samples from the same individual over time,” USDA National Wildlife Research Center biologist Sarah Bevins, who wasn’t involved in the study, tells National Geographic. “Capturing a cougar even one time is not a trivial effort.”

One of the cougars in the study, dubbed M21, was tested four times in a five-year period. The first two tests were negative, but the third was positive, which meant that M21 had fought off a plague infection. A year later, another test came back negative—the antibodies had faded away. Eventually, M21 died in 2012, probably because of exposure to rodenticide.

The researchers didn’t find a connection between age or sex of the big cats and their likelihood to contract the plague, raising questions for future research. It’s possible that the bacteria reaches mountain lions through their prey.

Constant screening of cougars for Y. pestis could help humans anticipate plague outbreaks, too. About 3,000 cougars are killed legally in the U.S. each year and inspected by authorities, providing an opportunity to check for the bacteria.

Three of the cougars in the study died of the plague in 2006, only two years before a teenage Boy Scout caught the same disease in the same region. Per the Wyoming News, the Scout made a full recovery. But in 2007, Colorado mountain lion researcher Eric York contracted the plague during a necropsy and died of the infection.

"Plague is very much a part of the landscape in the western U.S., and wild animals are frequently exposed to it," Bevins tells National Geographic. "[While] human plague infections are still relatively rare…untreated plague infections are still just as deadly as they were 500 years ago.”