One day in 2009, twenty-four-year-old Susannah Cahalan woke up alone in a strange hospital room, strapped to her bed, under guard, and unable to move or speak. A wristband marked her as a "flight risk," and her medical records--chronicling a monthlong hospital stay of which she had no memory at all--showed hallucinations, violence, and dangerous instability. Cahalan tells the astonishing true story of her inexplicable descent into madness and the brilliant, lifesaving diagnosis that nearly didn't happen. A team of doctors would spend a month--and more than a million dollars--trying desperately to pin down a medical explanation for what had gone wrong. Then, at the last minute, celebrated neurologist Souhel Najjar joined her team and, with the help of a lucky, ingenious test, saved her life. He recognized the symptoms of a newly discovered autoimmune disorder in which the body attacks the brain, a disease now thought to be tied to both schizophrenia and autism, and perhaps the root of "demonic possessions" throughout history.