He's talking about the case the The Exorcist movie was based upon. In real life the possessed was a boy. To add a little aside to the story, when I was at the Carmelite Monastery, the Novice Mistress told me that the boy had been hospitalized during the possession. Her mother was a nurse there at the time. The doctors and nurses who attended to him witnessed objects flying around the room, the bed shaking, foul odors, and other strange phenomenon. The patients who used the room afterwards hadn't known anything about the previous occupant, but they could smell the foul odor, saw strange stuff, felt a dark presence, and quickly begged to be moved to a different room. The hospital's solution was to turn the room into a storage facility, but that didn't last long. Strange stuff continued and the staff refused to go in there. The hospital eventually closed off the room permanently.
From what I hear quite a few people who are locked up are in reality possessed rather than mentally ill as such. I t mus tbe Halloween because I found another scarey one.