Exorcism
In Catholic Christianity, exorcisms are performed in the name of Jesus Christ. A distinction is made between a formal exorcism, which can only be conducted by a priest during a baptism or with the permission of a bishop, and "prayers of deliverance" which can be said by anyone.
The Catholic rite for a formal exorcism, called a "Major Exorcism", is given in Section 11 of the Rituale Romanum. The Ritual lists guidelines for conducting an exorcism, and for determining when a formal exorcism is required. Priests are instructed to carefully determine that the nature of the affliction is not actually a psychological or physical illness before proceeding.
The Exorcist (novel)
Recent investigative research by freelance journalist Mark Opsasnick indicates that Blatty's novel was based on an actual 1949 exorcism of a young boy from Cottage City, Maryland, whom Opsasnick refers to using the pseudonyms Robbie Mannheim and Roland Doe. The boy was sent to his relative's home on Roanoke Drive in St. Louis where most of the exorcism took place.