Answer by James Fisher:
The nexus of real-life events depicted in The Exorcist was indeed Saint Louis University (SLU), a Jesuit school, where I happen to be a professor. I've traded on my SLU connections to run down some of the elements in this story.
Yes, it was a boy, who was "possessed." He was a 13-year-old kid from Maryland and not the 12-year-old girl depicted by Linda Blair in the movie.
The train of events evidently got underway with some urgency in January of 1949. The family reported disturbing events surrounding their youngster: scratching sounds emanating from the walls, dripping water, the unexplained movement of objects, including the boy's mattress. A hypothesis emerges: the boy is plagued by the spirit of a recently deceased aunt, who according to family lore introduced the boy to the Ouija board.
It turns out that the boy and his family are not Catholics, so they hie themselves to a Lutheran minister with something of a reputation for things paranormal. The Reverend Luther Miles Schulze doesn't mince words, delivering an earnest piece of advice: "Go to a Catholic priest; the Catholics know about this kind of thing."
The family follows this advice and go to a local (still in the Maryland area) priest, who offers them some holy water and candles, but little else on this first visit. The priest, E. Albert Hughes, subsequently asks the archbishop of Washington, D.C., for permission to perform an exorcism on the boy. It's unsuccessful.
Things get weirder. Unexplained scratches appear on the boy's body, culminating with the word "LOUIS" emerging in relief along his rib cage. The family interpret this as a sign to take the boy to St. Louis, where they have relative attending, yes, you guessed it, Saint Louis University.
Enter the Jesuits. Several of them. The SLU student contacts her professor Father Bishop, who brings in his friend, another Jebbie and World War II combat veteran, William Bowdern, S.J, who also happens to be the pastor of St. Francis Xavier College Church. Paul Reinert, another Jesuit and then President of SLU gets consulted, as does St. Louis Archbishop Joseph Ritter. An exorcism it will be. According to the Roman Ritual. And Father Bowdern will do it. Other priests are enlisted, as are the Alexian Brothers.
Here's an account from a SLU publication, Universitas:
The process ended up taking more than a month, during which Bowdern fasted…[F]amily members participated in or witnessed the rite, which always began in the evening.
"The pattern was that the boy would act normally during the day, and then he would put on his pajamas and go to bed, and go into a trance and start screaming and yelling and acting wild," said [Thomas B.] Allen [author of Possessed: The True Story of an Exorcism]….
The exorcism continued on almost a nightly basis, even though the boy seemed to be getting worse. The priest asked his family for permission to teach him about Catholicism and convert him as a way to strengthen the fight against the supposed demonic possession. As he got closer to conversion and making his first holy Communion, his episodes become increasingly violent.
On April 18, the day after Easter Sunday, the exorcism appeared to have succeeded. An entry from Bishop's diary reads: "Since Monday at 11 p.m. there have been no indications of the devil."
The boy, whose precise identity is not public knowledge, left St. Louis and never experienced anything like demonic possession again. He married in 1970 and has raised a family.
More about this story is available in Allen's book, cited above, and in Saint Louis University's Universitas, Spring 2014 issue: