People with Cotard's Delusion believe they're on the wrong side of The Walking Dead TV series, meaning they think they're actually dead. How is it possible to treat them? Can you "prove" to a patient that they're still alive?
In 1880, Doctor Jules Cotard had the first of a series of what I can imagine were very frustrating conversations with patients in his care. His main patient was Madame X, who at first believed that certain parts of her body didn't exist and didn't believe that she needed to eat. She said that her soul was already stolen by the devil, that she was therefore already dead and couldn't die. She died of starvation. Over the ensuing century and a half many people have been diagnosed with Cotard's Delusion. Their delusions cover many angles of a similar theme; they believe that parts of them are dead, rotting, or missing, or they simply believe that they, as dead people, cannot be killed.
One 53-year-old Filipino woman was admitted to a psychiatric hospital, resisting all the way because she told doctors that she could smell her flesh rotting and she should be taken to the morgue. A man in Iran kept insisting that he was dead, that he was also a werewolf, and that his children had turned into sheep. Two different Chinese women believed that their guts – heart, liver, or stomach - had been taken out and that their insides were just filled with water. (This is a common delusion, though most people only think that their internal organs are rotting, not entirely removed.)
The more famous, and classic, cases of Cotard's Delusion are rare. One 46-year-old man believed that his body had been turned immaterial, and he was now a ghost. He stopped eating, since he no longer needed to eat to live, and lost 30 pounds in two months. He was admitted to the hospital in order to keep him from starving to death. Another man was admitted after a motor cycle accident left him convinced he was dead, and a trip to South Africa with his mother left him convinced that he had gone to hell (because of the heat, not the mother). He believed his sleeping mother's spirit was giving him a tour of hell.
Cotard's Delusion seems to be an aspect of larger mental illness. Most Cotard's patients are either schizophrenic, severely depressed, or bipolar. Madame X had the misfortune to have the syndrome when psychiatry was in its infancy (if not its actual gestation), but today Cotard's sufferers have a better outlook. Sadly, this is an area where talking rarely does anything to help; people cannot be convinced that they're still alive. One the plus side, since they're dead, they rarely fear any risks of treatment. Antipsychotics do a lot of good, as do antidepressants. (Most walking corpses aren't that cheerful.)
Fittingly, electricity seems to be a very good way of bringing people back from the dead. One woman, who believed she was a corpse for seven years, stated that she was alive again after the second round of electroconvulsive therapy. Another man, who had lesions on his brain and who claimed that his stomach had disappeared, felt no change after getting medication, but responded to electroconvulsive therapy. Shock treatment isn't the brutal thing we see in old movies. Patients are anesthetized, given brief shocks, and then woken up again. Cotard's patients often feel better after only a few treatments, and start socializing, taking care of themselves, and generally admitting that they're alive.
(I don't know whether, after the therapy, the doctors use the fact that patients are unconscious to scream "It's alive! It's aliiiiiiive!" I just know I would.)