You must have probably heard the phrase, the living dead. Well, there is one mysterious medical condition which actually makes a person think he or she is dead. People suffering from this mental illness usually deny their own existence, or the existence of a certain body part, or the existence of a portion of their body. 

Cotard Delusion aka the Walking Corpse syndrome is a rare mental condition where people believe they are either dead or non-existent.

Source: Hollywood Reporter

It is an ultra rare condition named after French neurologist, Jules Cotard. In 1880, a woman visited the neurologist as she believed she had 'no brain, no nerves, no chest, no stomach, no intestines. She said that 'she was nothing more than a decomposing body' and that 'she had no soul' and she had 'no need to eat'. Mademoiselle X, as Cotard dubbed her in his notes, later died of starvation.

Source: Wikipedia

This bizarre syndrome is usually associated with denial of self existence as a result of anxiety and loss of reality. In 2013, a woman named Esmé Weijun Wang had apparently developed it when she was in a flight from London to San Francisco. She said to The Washington Post

I was convinced that I had died on that flight, and I was in the afterlife and hadn’t realised it until that moment.

Another such case involved Graham Harrison, a man who had attempted suicide 9 years before he was interviewed by New Scientist in 2013. He said - 

When I was in hospital I kept on telling them that the tablets weren’t going to do me any good ’cause my brain was dead. I lost my sense of smell and taste. I didn’t need to eat, or speak, or do anything. I ended up spending time in the graveyard because that was the closest I could get to death.

During the same interview, Graham's doctor revealed that his brain function resembled that of someone during anesthesia or sleep. However, while Graham came out of it with the help of psychotherapy and drug treatment, other such patients have died from starvation over the years. Some have even resorted to pouring acid on themselves to escape from it.

Source: Mental Floss

How can someone not exist if they are cognitively aware of their non-existence? The answer to this is still unknown. This disorder is often linked to bipolar disorder, depression and/or schizophrenia depending on the patient's age. However, its cure is still an uncharted territory for the medical universe. Also, there are no formal estimates on the number of people affected by the Cotard Delusion.