A part of the "horror" of the whole phenomena surrounding The Exorcist is that a legend has grown up around the making of the film and the so called "curse".
There were several on set accidents during the production including an unexplained fire on set, cast and crew deaths, medical diagnoses of illnesses, the list goes on. I'm sure if they had been filming a romantic comedy none of this would have been attributed to the films subject matter itself but rather coincidence and general life.
This is a legend that had been encouraged by Billy Friedkin (director) and some of the cast. After all, it only enforces the idea that somehow The Exorcist is MORE than the sum of its parts, that real evil exists in the footage of this movie.
Some real priests played the role of supporting cast members to give it an air of authenticity.
Linda Blair was actually badly injured on set during the bed levitation scene, as she was rigged up to a harness which was violently jerked to make it looked as though she was being thrown about. She was screaming in real life for Friedkin to stop (if you listen to the footage closely you can hear her scream "BILLY" but it's lost in the general noise and screaming of the scene.)
Interestingly Max Von Sydow who played the older priest was vocal in denying all of the stories surrounding the movie, and very dismissive of the "curse" while Ellen Burstin (who played the mother) encourages these stories in many interviews about the making of the film.
Also pertinent is the fact that the story by William Peter Blatty is VERY VERY loosely based on a true story of an apparent possession of a young boy in South America. Although this has to be taken with a massive pinch of salt as always, these "legends" are embellished over the years so that the original facts become very dubious.
The ambulances and medics stationed at the cinema were for a large part actually part of the planned media circus surrounding the film's release. The stories of people fainting, vomiting and having seizures in the aisles was a kind of self fulfilling prophecy and became like a contagious hysteria among young people. I'm not saying it DIDN'T happen but again, it's a legend that has grown up around the film and one that Friedkin and Co are only too happy to foster and encourage.
The film was taken off the shelves in 1988 (home VHS tapes) but actually continued to be shown in certain cinemas with special showings, so it was never outright banned. Almost all of the hype surrounding the film was generated by the studio on an effort to sell the film, and it worked.
Personally I like the film. As a Christian I suppose it affects me in a way that might not affect someone with no religious beliefs, so that adds another layer of interest. I don't think the movie has held up particularly well over the years, and the subsequent sequels were DIRE and should be relegated to the cinematic bin.