As for the supposed "true story" nature of the Exorcist, the case upon which it is loosely based actually involved a 13 year old boy in 1949. The boys family lived in Washington DC. They started hearing scratching noises in their walls and got an exterminator thinking it was rats or something but it didn't stop the noises. Not long after the boy started acting bizarrely, and welps appeared on his torso. The family decided to leave and visit relatives in the midwest, but the boy's behavior turned for the worse and he began acting hateful and more bruises and cuts and things appeared on his body. One of his family members was a student at St. Louis University and sought help from one of the Jesuit professors. Eventually the boy was brought to campus, kept in a room on the top floor of Verhagen Hall. Apparently they had to restrain him. At some point while he was on campus, the the words "NO" and "LOUIS" appeared scratched into his chest. Several Jesuit Preists, including Father William Bowdern partook in an exorcism. Before they began the boy was moved to Alexian Brother Hospital a few miles south of SLU. The exorcism ceremony, which is supposed to take 10 minutes, took several hours do to difficulty with the boy. What ever the case may be as to the cause, the boy snapped out of his behavior claiming that St. Michael saved him.
Personally I don't believe in possession, but the Jesuits at SLU certainly do. I'm a student at Saint Louis University currently and while its not something they advertise, SLU in no way denies the events said to have taken place on campus or involving their employees. The room in Verhagen Hall was walled off for decades, first with drywall but later a full wall was built when the whole building was restored in the late 80s. They had to do some work a few years back though because they had some problems with the room. Apparently through some old chimney pipe or something there was access to the outside and several dozen pidgeons and squirrrels had made their way into the room and died.
Several of Father Bowdern's personal effects including a crucifix they claim was used in the exorcism reside in SLU's art museum, the top floor of which is devoted to Jesuit history.
The ward of Alexian Brothers Hosipital was not in active use even in '49 when the boy was taken there. It too was walled off after the events and was actually torn down about 15 years ago.
It's all rather strange. Something certainly happened. It is a fact the boy was brought here. Even if it all was a complete myth, it certainly wouldn't be the most ridiculous thing treated as fact at this Catholic school.