Victims of an uncommon neuropsychiatric disorder known as Cotard delusion, or walking corpse syndrome think they are zombies.
People who suffer from this disorder believe they have died, that their flesh is rotting, or even that they have lost a vital organ, reports Msnbc.com.
The unusual illness has been linked to bipolar disorder and schizophrenia.
Victims think their bodies don't even have blood and some test their mortality by trying to take their own lives, reports the New York Daily News.
Current treatments involve drugs and/or electroconvulsive therapy. The latter involves placing electrodes on top of a patient's head and administering electricity - purposely causing seizures.
Unlike most other disorders, patients with walking corpse syndrome don't actually have anything at all wrong with them. They just don't know they're okay.
The illness is described in the American Psychiatric Association's Diagnostic and Statistical Manual of Mental Disorders (DSM), and was first described by French neurologist Jules Cotard back in 1882.