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(CNN)A small group of nuns and priests met the woman in the chapel of a house one June evening. Though it was warm outside, a palpable chill settled over the room.

As the priests began to pray, the woman slipped into a trance -- and then snapped to life. She spoke in multiple voices: One was deep, guttural and masculine; another was high-pitched; a third spouted only Latin. When someone secretly sprinkled ordinary water on her, she didn't react. But when holy water was used, she screamed in pain.
"Leave her alone, you f***ing priests," the guttural voice shouted. "Stop, you whores. ... You'll be sorry."
You've probably seen this before: a soul corrupted by Satan, a priest waving a crucifix at a snarling woman. Movies and books have mimicked exorcisms so often, they've become clichés.
The 1973 film "The Exorcist" shaped how many see demonic possession.
The 1973 film "The Exorcist" shaped how many see demonic possession.
But this was an actual exorcism -- and included a character not normally seen in the traditional drive-out-the-devil script.
Dr. Richard Gallagher is an Ivy League-educated, board-certified psychiatrist who teaches at Columbia University and New York Medical College. He was part of the team that tried to help the woman.

Fighting Satan's minions wasn't part of Gallagher's career plan while he was studying medicine at Yale. He knew about biblical accounts of demonic possession but thought they were an ancient culture's attempt to grapple with mental disorders like epilepsy. He proudly calls himself a "man of science."
Yet today, Gallagher has become something else: the go-to guy for a sprawling network of exorcists in the United States. He says demonic possession is real. He's seen the evidence: victims suddenly speaking perfect Latin; sacred objects flying off shelves; people displaying "hidden knowledge" or secrets about people that they could not have possibly have known.
"There was one woman who was like 90 pounds soaking wet. She threw a Lutheran deacon who was about 200 pounds across the room," he says. "That's not psychiatry. That's beyond psychiatry."
Gallagher calls himself a "consultant" on demonic possessions. For the past 25 years, he has helped clergy distinguish between mental illness and what he calls "the real thing." He estimates that he's seen more cases of possession than any other physician in the world.
There was one woman who was like 90 pounds soaking wet. She threw a Lutheran deacon who was about 200 pounds across the room. That's not psychiatry. That's beyond psychiatry.

Dr. Richard Gallagher, psychiatrist, professor and demonic possession "consultant"

"Whenever I need help, I call on him," says the Rev. Gary Thomas, one of the most famous exorcists in the United States. The movie "The Rite" was based on Thomas' work.
"He's so respected in the field," Thomas says. "He's not like most therapists, who are either atheists or agnostics."
Gallagher is a big man -- 6-foot-5 -- who once played semipro basketball in Europe. He has a gruff, no-nonsense demeanor. When he talks about possession, it sounds as if he's describing the growth of algae; his tone is dry, clinical, matter-of-fact.
Possession, he says, is rare -- but real.