Jeannine Fleegle reached into a garbage bag, pulled out a severed deer head and placed it on a folding table.

“This is no one’s favorite time of year,” Fleegle said, picking up a scalpel.

It was a chilly morning, and Fleegle, a wildlife biologist for the Pennsylvania Game Commission, was with a half-dozen other scientists in the small town of Bolivar. Covered in head-to-toe white Tyvek suits, they were surgically extracting hundreds of brain-stem samples from deer killed by hunters during the state’s rifle season. The samples would be analyzed for signs of a deadly pathogen.

The formal name of the ailment is chronic wasting disease, or CWD. But its effects on deer, elk and other cervids – weight loss, stumbling, listlessness and certain death – have inspired a creepier colloquial name: zombie deer disease.

More than 50 years after it was first detected, the disease is now spreading rapidly. Last winter, Tennessee became the latest of 24 states to report CWD infections, which have also been found in two Canadian provinces, Norway, Finland and South Korea. Now, as it strikes animals across a widening territory, concern is growing among scientists and public health officials that the disease might leap to humans.

CWD is a transmissible spongiform encephalopathy, another of which did jump species: Mad cow disease. In humans, mad cow disease is known as variant Creutzfeldt-Jakob disease, and it has killed more than 220 people worldwide since the 1990s. Some experts said that in a nation with an estimated 10 million deer hunters harvesting 6 million deer a year and eating many of them, it may be just a matter of time before chronic wasting makes its way to us.

Both CWD and mad cow are thought to be caused by proteins that malfunction and misfold, called prions. There is no known cure or treatment for prion diseases.

“Last year, we estimate that as many as 15,000 carcasses may have been served to people that were CWD-positive,” said Michael Osterholm, director of the Center for Infectious Disease Research and Policy at the University of Minnesota. “And what people don’t understand with prion-related conditions like this is that time changes the risk.”

There are thought to be eight strains of CWD, and every time one goes through an animal, Osterholm said, there’s a chance it might adapt in a way that allows it to pass into humans.

This threat is adding urgency to Fleegle’s work. On the same morning she dug into deer brains, biologists at seven other stations across the state were doing the same. Their mission is to establish and maintain “disease management areas,” where hunting remains permitted but with additional restrictions on carcass handling and processing. The goal is to halt CWD’s spread.

A distinctive odor of wet fur and death loomed as Fleegle sliced a deer’s neck down to the last vertebrae, exposing the brain stem beneath. “You core it like an apple,” she said, swirling the instrument in the cavity to detach nerves holding the stem in place. Next, she plucked a thumb-sized piece of creamy, white tissue and dropped it into a jar of formalin, which preserves the sample for testing.

The disease can spread from animal to animal via saliva, blood, urine and feces, where prions build up. Inside the body, the brain, spinal column and spleen all serve as reservoirs for the infected prions, which makes them good areas for diagnosis.

Fears about CWD’s threat to new species rose last year, when a decade-long study provided the first evidence the disease could develop in primates under certain conditions. Scientists successfully gave CWD to macaque monkeys by injecting it into their brains and by feeding the animals infected brain material from a deer. Some monkeys even developed infections after eating tainted venison.

The finding was alarming because macaque DNA is similar to our own, said Stefanie Czub, a virologist at the University of Calgary and lead researcher of the study. But Czub said the results should be interpreted with caution.

“It’s not really like you put it into a macaque and boom, they come down with the disease,” she said.

While a hunter might come in contact with a deer that has one CWD strain, Czub’s team hit the macaques with multiple strains at once to test the limits of what’s possible.

Even so, the Centers for Disease Control and Prevention has taken a better-safe-than-sorry approach when advising hunters.

“We urge you to strongly consider having your animal tested if it’s harvested from an area where chronic wasting disease has been found,” said Ryan Maddox, a CDC epidemiologist.

If it tests positive, don’t eat it, the CDC said.

That advice might seem like it would go without saying. But Fleegle said she’s heard hunters say they planned to dine on infected deer. “The decision to eat any harvested animal lies with the hunter,” she said. “We are not a food safety agency.”

Pennsylvania and many other states now enforce restrictions on transporting deer killed in areas where CWD is established, and that’s important because deer season amounts to a sort of pilgrimage for many hunters. During the 2016-17 season, for example, people from 49 other states traveled to Wisconsin to hunt deer, despite nearly every county in the state testing positive for CWD or being adjacent to one that has.

Records show 32,000 deer were killed in such areas. Although it is not known how many were infected or how many carcasses were taken out of Wisconsin, the potential for spread is clear.

The problem with moving diseased carcasses is that CWD prions have a remarkable ability to persist in the environment even after a carcass has been discarded and withered away, said Krysten Schuler, a wildlife disease ecologist at the Cornell University College of Veterinary Medicine.

In experiments, prions have remained present and infectious two years after a carcass decays. If CWD prions are as hardy as the ones that cause a disease in sheep called scrapie, that timetable might be even longer.

“They’ve done things with scrapie where they actually buried brain material in the soil and dug it up 16 years later, and it still caused disease,” Schuler said.

While harvested deer are one danger, the disease can also be spread by moving farmed deer, either for big-game hunting, breeding or petting zoos. The pathogen even seems to have made the voyage across the Pacific as a stowaway inside captive Canadian reindeer destined for a Santa’s Village in South Korea.

Shawn Schafer, executive director of the North American Deer Farmers Association, said he thinks cervid farming and CWD have been unfairly linked. Deer farms are often the “canary in the coal mine” for identifying new infections, he said, because they test every dead animal, whereas states sample a small proportion of the wild population.

Schafer said he’s more concerned about other wildlife diseases, such as West Nile virus, than CWD.

“As a rule, we don’t eat sick animals,” he said. “But do I test all the deer that I hunt? No. I’m sure many of them have been positive, and I’m just not worried about it.”