"The Exorcist," is based on the story of a 14-year-old boy known as Roland Doe, who began exhibiting strange behavior in 1949. His family reported furniture moving on its own, scratches all over Doe's body, and loud, disembodied voices.
After a cross-country move, and no change in Doe's strange behavior, his family enlisted the help of the Catholic church. One of the priests involved in the Doe case, Father Raymond Bishop, kept a diary. One of his entries stated "at midnight, the Fathers planned to give (Roland) Holy Communion, but Satan would have no part of it. Even while the institution of the Blessed Sacrament was explained, his body was badly scratched and branded. The word 'HELLO' was printed on his chest and thigh."
Eventually, after multiple attempts, the "demon" was exorcised and Doe returned to normal. He never spoke of the incident publicly.