But the threat is not over, experts say. Not only do sporadic cases of mad cow disease still occur in cattle, but similar pathogens lurk in animals across the globe, posing a potential threat to human health, and the economy.

So-called chronic wasting disease (CWD)—colloquially referred to as “zombie deer disease,” a similar condition that affects cervids like deer, elk, reindeer, and moose—has been detected in 32 American states, three Canadian provinces, South Korea, Finland, Norway, and Sweden. In some U.S. counties, over 80% of deer are infected.

Experts worry that the progressive, fatal illness could spill over to humans in a fashion similar to mad cow disease. The resulting economic and public health crisis could be worse, however, according to experts at the University of Calgary in Canada. That’s because mad cow disease affected a population relatively easy to control: farmed cattle. CWD, on the other hand, can affect both farmed and free-range animals, making total control impossible.

Society simply isn’t prepared for such a scenario. But one man—epidemiologist Michael Osterholm, director of the University of Minnesota’s Center for Infectious Disease Research and Policy (CIDRAP)—is attempting to change that. His organization recently launched an effort to prepare for a possible spill over to humans or farm animals humans might consume.

“There’s no evidence yet that we’ve seen crossover into humans, but what if it were to happen?” he tells Fortune. “There’s enough concern that it could happen that we have to be prepared. If we find 25 years from now that all this work we did was unnecessary, that would be the greatest victory of all.”

The international think tank, funded by the Minnesota legislature, is composed of 67 experts from seven countries, including representatives from federal agencies like the U.S. Centers for Disease Control and Prevention and National Institutes of Health, universities, and tribal communities. It will tackle topics like public health and animal surveillance, diagnostics, lab capacity, and planning and response for the disease, for which there are no easy or rapid tests—or vaccines or therapeutics.

The group has only recently begun to meet, so its recommendations to the scientific community are forthcoming. In the meantime, here’s what you need to know about this very real threat to human and animal health.

What is chronic wasting disease?

Chronic wasting disease (CWD) is an infectious condition that affects deer, elk, reindeer, and moose. Animals become infected by contact with the feces, saliva, blood, or urine of infected animals, or through soil, food, or water contaminated with such fluids. Scientists believe that environments contaminated with the disease—perhaps the soil where a deer who died of CWD decomposes—remain infectious for years, if not decades.

Symptoms in animals include drastic weight loss (wasting), stumbling, listlessness, and other neurologic symptoms.

Has chronic wasting disease occurred in humans?

No human cases have yet been reported. Similar diseases have, however, been reported in humans.

What type of pathogen causes chronic wasting disease?

Chronic wasting disease is caused by infectious proteins called prions. All diseases caused by infectious proteins are called prion diseases. Such diseases are untreatable neurodegenerative conditions that can incubate in an animal or person for decades before leading to rapid deterioration and death about a year after the onset of symptoms.

How can a protein cause an infection?

Until recently, it was thought that only organisms like viruses, bacteria, fungi, and parasites functioned as pathogens. In the 1980s, neurologist and biochemist Dr. Stanley Prusiner identified the first infectious protein. In 1997, he won a Nobel Prize for the discovery.

Prion diseases occur when misfolded proteins cause normal proteins to misfold themselves by mechanisms yet unknown, thereby spreading disease. Misfolded proteins can accumulate into toxic clumps that damage the brain and eventually lead to death.

How might a human contract chronic wasting disease?

Researchers aren’t yet sure, but they worry that the disease could be caused by eating the meat (or venison or sausage created from the meat) of infected deer, just as eating the meat of infected cows causes a version of mad cow disease in humans.

In the case of CWD, infectious proteins are contained not only in the brain of deer, but in the peripheral nerves of their muscles, according to Osterholm. “Cooking the meat actually concentrates the prions and makes it worse,” he says.

It’s also possible that CWD spills over into farm animal populations like pigs and cows, with a much larger impact on the food supply and economy. Already, certain countries embargo U.S. pasture grasses from import, Osterholm says, as prions from dead animals with such diseases can take up in the root structures of grass and remain there for decades.

It’s also theoretically possible for humans to contract prion diseases by eating meat processed on contaminated equipment, experts say.

What can humans do to avoid potentially contracting chronic wasting disease?

Those who hunt in areas where CWD has been detected in deer should “get your deer tested and don’t eat it if it’s positive,” Debbie McKenzie—a biology professor at the University of Alberta in Canada and an expert on CWD—tells Fortune. “If you don’t eat infected deer,” your risk of contracting the disease is “infinitely lower.” 

She hears frequently of hunters whose deer test positive for the disease but eat the meat anyway. “I wish I could say it’s five or six people, but it’s not,” she says. “It happens frequently.”

“Maybe the risk of transmission is zero or very close to zero,” she adds. “Personally, I would prefer not to take the chance.”

Would we already know if chronic wasting disease spilled over into the human population?

Not necessarily. All prison diseases have long incubation periods that last decades—sometimes upwards of a half a century. Thus, it’s possible that humans have already contracted the condition but won’t experience symptoms for years. Once people become symptomatic, the disease is always lethal, usually within a year of symptom onset.

If cases of symptomatic disease began hitting the public health community’s proverbial radar, say, tomorrow, officials wouldn’t have any idea of the exact cause of such cases (deer meat? meat of another animal?) and when infection occurred (decades ago? sometime in the past few years?)—issues CIDRAP’s working group aims to tackle.

In a 2022 study published in the journal Acta Neuropathologica, researchers from the University of Calgary noted that mice—biologically similar to humans—infected with CWD passed infectious prions in their stool. Infected cervids do this, but humans with prion disease have never been known to do so, the paper’s lead author, researcher Samia Hannaoui, tells Fortune.

But Hannaoui, a prion disease researcher at the University of Calgary, has seen CWD-infected mice and rhesus monkeys—both biologically similar to humans—display unusual symptoms and disease progression. Because of this, she worries that public health officials may not immediately recognize that people infected with CWD have the condition.

“It makes us think it might happen in humans, and we might miss it,” she says. “We’re looking for something very specific, and it might manifest in a totally different way.”

What are some other prion diseases?

There are a number of human prion diseases as well. Creutzfeldt-Jakob disease (CJD) occurs when infectious proteins accumulate in the brain. Just how the vast majority of patients develop the condition is unknown. But between 5% and 15% of patients have errors in the PRNP gene that lead to the condition. Each year about 350 cases are diagnosed in the U.S., usually in people in their sixties.

Aside from CWD, prion diseases among animals include bovine spongiform encephalopathy (BSE), also known as mad cow disease; scrapie, the same virus in sheep; transmissible mink encephalopathy; feline spongiform encephalopathy; and ungulate spongiform encephalopathy.

Other human prion diseases include Gerstmann-Straussler-Scheinker disease and fatal familial insomnia, both of which, like CJD, are extremely rare and caused by errors in the PRNP gene as well. Most cases are inherited, though sporadic cases can occur.

When BSE (mad cow disease) is passed to humans who eat meat from sick animals, it’s known as variant Creutzfeldt-Jakob disease (vCJD).

What symptoms would chronic wasting disease cause in humans?

Since no spillover events have been detected, scientists aren’t sure. However, symptoms of human prion diseases in general are similar to those in animals and include …

• rapid onset dementia
• difficulty walking
• involuntary muscle movements
• confusion
• mood changes
• hallucinations
• muscle stiffness
• impaired memory
• depression
• fatigue
• difficulty speaking
• invariably, eventual death

However, Hannaoui’s research team has also seen “whole body spasms” in mice they infected with CWD—a symptom not seen before in animals or humans with prion disease. What’s more, not all mice infected with CWD died—an unusual finding in the setting of what should be an invariably lethal disease.

Can prion diseases pass from human to human?

Prion diseases can be passed among humans in rare cases, such as through the transplant of corneas, blood transfusions, and via contaminated medical equipment.

If CWD spilled over into humans, however, it might transmit more easily, Hannaoui warns. Her team found that CWD-infected mice pass prions in their stools. Infected cervids do this, but humans with prion disease have never been known to do so.

The finding could mean that humans with CWD pass infectious prions in their fecal matter, according to Hannaoui. That would mean the disease could be passed from person to person via fecal-oral transmission—like polio, E. coli, typhoid, and cholera—providing a route for easy spread and possible endemicity in humans.

“This is our main concern, actually,” she says.