“Zombie disease” most commonly refers to chronic wasting disease (CWD), a fatal brain disease that infects deer, elk, moose, and reindeer. It earned the nickname because infected animals lose dramatic amounts of weight, stumble around with blank expressions, drool excessively, and lose their natural fear of people. The disease is caused by misfolded proteins called prions, and there is currently no cure, no vaccine, and no treatment. Every infected animal dies.
Why It’s Called Zombie Disease
The “zombie” label comes from how visibly disturbing the symptoms are. An infected deer may stand motionless for long periods, appearing completely unaware of its surroundings. As the disease progresses, animals develop a gaunt, skeletal appearance from severe weight loss, stumble or lose coordination, drool constantly, drink and urinate excessively, and let their ears droop. They stop fleeing from humans or predators. The combination of an emaciated body and that vacant, listless behavior is what prompted media outlets and wildlife officials to start calling it “zombie deer disease.”
These symptoms can take months or even years to appear after infection. During that long silent period, the animal looks and acts perfectly healthy while still being capable of spreading the disease to others.
How Prions Destroy the Brain
CWD isn’t caused by a virus or bacteria. It’s caused by prions: normal proteins that exist naturally in the body but have folded into the wrong shape. Once a misfolded prion enters an animal, it acts like a template, forcing nearby healthy proteins to rearrange into the same abnormal shape. Those newly misfolded proteins then convert more, creating a chain reaction.
The misfolded proteins pile up in the brain over time, physically destroying tissue. As brain cells die, the animal progressively loses control over its body and behavior. Because prions contain no DNA, the immune system doesn’t recognize them as a threat, which is why the body can’t fight back and why no treatments exist.
Where CWD Has Spread
As of 2025, CWD has been confirmed in free-ranging deer and elk across 543 counties in 36 U.S. states, spanning every region of the country. States with confirmed cases range from Colorado and Wyoming (where CWD was first identified in the late 1960s) to more recent detections in California, Florida, Georgia, and New York.
Internationally, the disease has been found in Canada, Norway, Finland, and Sweden, primarily in reindeer and moose. A small number of cases were also imported into South Korea through farmed elk.
One reason CWD keeps spreading is that prions are extraordinarily tough. Unlike viruses, they aren’t alive, so they can’t really be “killed.” Prions shed into the environment through saliva, urine, feces, and decomposing carcasses remain infectious in soil and water for years. An area where a CWD-positive animal lived or died can continue infecting new animals long after the original host is gone. This environmental persistence makes containment extremely difficult for wildlife agencies.
Can Humans Get It?
This is the question most people actually want answered, and the current evidence is reassuring. No case of CWD has ever been confirmed in a human being. A 2024 study published in the CDC journal Emerging Infectious Diseases tested whether CWD prions could infect human brain tissue grown in the lab. Researchers exposed human cerebral organoids (miniature lab-grown brain models) to high concentrations of CWD prions from three different sources for a full week, then monitored them for up to 180 days. The CWD prions never converted human proteins or propagated in the tissue.
That’s significant because the same type of lab-grown brain tissue readily became infected when exposed to the human form of prion disease (Creutzfeldt-Jakob disease), confirming the test was sensitive enough to detect infection. The results point to a strong species barrier between cervid prions and human biology.
The concern isn’t unfounded, though. A different prion disease, mad cow disease, did jump from cattle to humans through contaminated beef in the 1990s, causing a fatal brain illness. Because hunters in CWD-affected areas regularly handle and eat venison, scientists continue monitoring for any sign that CWD could make a similar leap. The CDC recommends that hunters in affected areas have their harvested deer or elk tested for CWD before eating the meat, and avoid consuming any animal that tests positive.
How Wildlife Agencies Are Responding
Because CWD can’t be treated and prions persist so stubbornly in the environment, management efforts focus on slowing the spread rather than eliminating the disease. The main strategies include widespread surveillance and testing of both wild and farmed deer populations, monitoring interstate and international movement of live cervids, and proper disposal of carcasses and contaminated materials to reduce environmental contamination.
Surveillance for CWD in farmed and wild cervid herds has been ongoing since the late 1990s. States with confirmed cases typically set up testing stations during hunting season where hunters can submit tissue samples. In some areas, wildlife agencies conduct targeted culling to reduce deer density in infection hotspots, since animal-to-animal contact is a primary route of transmission.
The Other “Zombie” Disease in Nature
CWD isn’t the only thing in nature that earns the zombie label. A parasitic fungus that infects carpenter ants is often called the “zombie ant fungus.” It works through a completely different mechanism but produces equally unsettling results.
The fungus grows into the ant’s muscle tissue, eventually occupying roughly 40% of the biomass in the insect’s mandibles and legs. The fungal cells physically separate muscle fibers, degrading them from the inside. This mechanical takeover forces the ant into a “death grip,” clamping its jaws onto a leaf or twig in a location that’s ideal for the fungus to release spores. The ant dies locked in place, and a fungal stalk erupts from its head to spread to new victims.
Interestingly, unlike some related fungi that invade the brain directly, this particular species doesn’t penetrate the ant’s nervous tissue until after death. Instead, it appears to control behavior indirectly, likely through chemical signals and the physical destruction of muscle, essentially hijacking the body while leaving the brain structurally intact. It’s a fundamentally different process from CWD, where the damage is centered entirely in the brain, but both earn the “zombie” comparison for the same reason: the host appears to lose all control over its own body.