I don't believe in demonic possession, but I can understand why people dreamed up the notion. Before we knew about mental illness, Tourette's , epilepsy, sleep disorders (and in some cases, acting out for attention), it seemed the best explanation. Fred is cursing, talking nonsense and being unFredlike? Some other entity must be manipulating him.
Science has sniffed out non-supernatural explanations for weird and obnoxious behavior, but some people still believe.
Peter Blatty's famous book, The Exorcist, boasts: "Based on a True Story."
Was it? Sorta.
There was an exorcism or two performed on a boy in 1949. The operative word here is "performed," as I don't think any demons got the boot. Details can be found here, but in a nutshell it seems the kid was scratching walls, shaking the bed, throwing things and acting nasty. On his own.
It's interesting that demons or "Satan Himself" get credit for possessions. Satan's a bad guy, right? Evil personified. So when he takes over a kid's body, is he killing people? Lighting fires? Robbing banks? Blowing up buildings?
No, he's busy scratching a wall and shouting bad words. Amateur.
My two theories on this.
1. Satan is, in fact, a bratty preschooler.
2. (More likely) Demonic possession is hooey.
Note: Ironically, as I drew the Captain Howdy in Diapers pic, a radio show I was listening to unexpectedly jumped into a segment with music from the famous movie. Coincidence? Probably. I'd be more impressed if the computor monitor started spinning.