Rare cases of demonic possession had happened in the United States, but they were not widely known until William Peter Blatty's bestselling 1971 book, The Exorcist. Blatty had heard about a case nearby while he was a student at Georgetown in Washington, DC. The real case was shrouded in misinformation, but the 13-year-old boy who inspired the book recovered and led a normal life. The demons, though, apparently lingered in the place where they were cast out of the boy, and the room was kept locked until the building was demolished.
When the movie opened in 1973 audiences were completely freaked out.