Zombies are so hot right now that they seem to be everywhere. The Walker hordes are featured heavily on "The Walking Dead," a show that debuted in 2010 and is arguably one of the most popular shows on television. Zombies also make an appearance in video games such as Call of Duty, and people are even pulling zombie apocalypse pranks. Essentially, zombies have become a mainstream way to suspend our disbelief and engage with the land of make-believe. However, they may be walking among us in real life thanks to a rare mental illness called Cotard’s Syndrome.

People with the condition are known to have any of a number of delusions that range from believing they have no organs, blood or body parts to insisting they are actually dead even though they are very much still breathing. In even more rare cases of what is also known as walking corpse syndrome, people with the condition consider themselves immortal. Although walking corpse syndrome is considered extremely rare, its exact prevalence is unknown. One Hong Kong study involving 349 psychogeriatric patients suggests the condition afflicts about 0.57 percent of the population. Another study conducted in Mexico in 2010, which involved 1,321 patients, found that 0.62 percent of the population had the rare mental health disorder. Perhaps unsurprisingly, this syndrome is often found in people with mood or psychotic disorders and medical conditions.

Cotard’s syndrome was first described by (and later named after) French neurologist Jules Cotard in 1880, Psychology Today reported. Cotard had a patient nicknamed Mademoiselle X who claimed she was missing several body parts and organs, including brain, chest, stomach and intestines. She also believed she was “eternally damned.” Given she denied having innards, Mademoiselle X didn’t see a need to eat, and soon died of starvation. Cotard said the condition was a form of delusion associated with severe depression, “marked psychomotor retardation, presence of anxiety symptoms and other depressive symptoms,” researchers said.

A more recent case of the condition involved a man named Graham who has been living with walking corpse for at least nine years. His condition emerged after he struggled with severe depression. Eight months after attempting to commit suicide by taking an electrical appliance with him into the bathtub, he told doctors he believed his brain was dead or missing, Medical Daily previously reported.

Symptoms of walking corpse syndrome can vary greatly. However, researcher Yamada Katsuragi and his colleagues published a longitudinal study in 1999 suggesting that the syndrome seems to have three stages: germination, blooming and chronic.

The germination stage is marked by hypochondriasis, or health anxiety, and severe depression. They may go to the doctor with vague complaints, as was the case with a 28-year-old housewife whom researchers refer to as Ms. S. A case report shows she went to an outpatient department complaining that her liver was “putrefying” or decaying and that her heart was “altogether absent.”

Before seeking medical help, Ms. S initially developed loss of sleep and a feeling of loneliness which are said to have progressed to loss of interest in her surroundings and daily household work — both classic signs of depression. This withdrawal is common among people who are severely depressed and could represent the germination stage of the syndrome for Ms. S. Graham also suffered from severe depression, as do many people with the condition.

The blooming stage is known as the time patients develop hallmark symptoms of walking corpse syndrome, such as denying they have certain body parts or that they even exist. These symptoms worsen in severity during the chronic stage and interfere with patients’ ability to go about their daily lives. They are known to begin to neglect their own hygiene and self-care, to harm themselves, and fail to recognize their own faces as well as those of others.

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For example, Graham, who no longer believed he had a brain or head, refused to eat, lost interest in smoking and being social because he saw “no point because I was dead.” Graham also made frequent visits to his local cemetery because he said “it was the closest I could come to death” and thought he would fit in there. "I didn't want to face people," he told the New Scientist magazine. "There was no point. I didn't feel pleasure in anything. I used to idolize my car, but I didn't go near it. All the things I was interested in went away."

Graham became the first person with walking corpse syndrome to undergo a brain scan. Doctors found activity levels in large areas of his frontal and parietal brain — areas involved in motor function, memory, and sensory information — were extremely low, resembling someone in a vegetative state.

“I’ve been analyzing PET scans for 15 years and I’ve never seen anyone who was on his feet, who was interacting with people, with such an abnormal scan result,” neurologist Steven Laurey, at the University of Liege in Belgium, told The New Scientist. “Graham’s brain function resembles that of someone during anaesthesia or sleep. Seeing this pattern in someone who is awake is quite unique to my knowledge.”

In addition, Scientific American attributes the failure to recognize people's faces to misfiring in the amygdala area of the brain. This may also explain why some people with this condition don’t believe they exist, because not recognizing one’s own face “can lead to a lack of association between their reflections or projected self and their own sense of self, leading to a belief that one doesn’t exist,” Scientific American reported.

Therapy along with antidepressants and antipsychotics have been shown to help control the symptoms as well as electroconvulsive treatment. In fact, a 2008 case report claims most cases of walking corpse syndrome are more responsive to electroconvulsive treatment than to pharmacological treatment. However, more research needs to be done on this condition because scientists are still in the dark about the cause of this disease and how to treat it.