In my 40+ year career in health care, I saw one person who was supposedly demon possesed. I was a 3rd year med student. The man was in his early 20s. His family said he’d been in good health and didn’t take drugs or use alcohol. There was no history of head trauma. In the last couple of weeks he unexpectedly had a sudden personality change. He became sullen and withdrawn. He stopped going to his job. He said he heard voices telling him to do terrible things. He began talking incoherently, and at times shouting or barking like a dog. He ate and drank very little. For the last several days, he was in bed, not moving, incontinent, eyes staring straight ahead, and seemingly unresponsive. His very religious parents were certain he’d been demon possessed. For nearly a whole day, his pastor, and other church members prayed at his bedside for his deliverance. When he stopped eating and drinking entirely, he was finally brought to the ER. A head CT scan was normal. (This was the pre-MRI era.) A spinal tap was negative for infection. Aside from dehydration, all his blood tests were in the normal range. The senior neurology and psychiatry residents examined him. It was obvious this young man had suffered an acute schizophrenic break, and was in a catatonic state. He was admitted to the psychiatry service, started on IV fluids, and given IM Haldol every 4 hours. By the next morning, he had woken up, was walking on the ward, drinking coffee, and talking coherently with the staff. I remember this so clearly because it was the most incredible turnaround in any patient’s condition that I’ve ever seen.
I suppose that demon possession can’t be ruled out with absolute certainty. But I can’t imagine what kind of supernatural demon can resist hours of prayer, but is chased away by a anti-psychotic drug.