For Mormons, the devil is a fallen angel. He rebelled against God and Christ in a pre-Earth life and persuaded one third of the other spirits in heaven to join with him. As fallen angels, they didn't get bodies. Now they want desperately to have one, so they try every way they can to jump into someone else's skin.
Sometimes these spirits have to be cast out of "afflicted persons by the power of faith and authority of the priesthood," wrote Bruce R. McConkie in his authoritative book, Mormon Doctrine.
The Church of Jesus Christ of Latter-day Saints believes that missionaries are particularly vulnerable. Thus, the army of men and women serving the church on two-year and 1 1/2 -year missions are forbidden to swim, which some LDS members attribute to the belief that Satan controls the water.
But LDS Church spokesman Scott Trotter put it in more practical terms.
"The church takes any necessary precautions to ensure the safety of its missionaries from natural disaster, public health threats or other potentially harmful situations," says Trotter.
"As a precaution, missionaries are advised not to swim during their missions."
Sometimes Mormon leaders and members spread cautionary tales about the dangers of engaging with evil spirits - stories repeated so often they enter the realm of folklore. The William A. Wilson Folklore Archives at Brigham Young University has scores of these stories, including several collected in the 2001 student project, "All Hell Let Loose: Missionary Devil Stories in LDS Culture."
There's the one about missionaries who encounter Satan when he answers the door at a home they visit. Or the tale of a potential convert levitating in a room full of spinning objects.
One of the most famous tales, told enough to be in the folklore category, is of the missionary who breaks his arm, then goes to a Christian revival to be healed. The non-Mormon revival leader heals his arm, but the missionary starts acting strange, so his fellow missionaries take him to the mission president who commands the evil spirit to leave. Almost immediately, the elder feels his arm break again.
Beyond folklore, though, many Latter-day Saints have had what they believe are real experiences with dark forces.
Mormon founder Joseph Smith described wrestling with the devil in his 1838 account of a vision of God and Jesus in a grove of trees. "Thick darkness gathered around me, and it seemed to me for a time as if I were doomed to sudden destruction," Smith wrote in the canonized version of the vision.
Those who encounter demonic forces are told to dispel them by drawing on their priesthood power in the name of Jesus Christ.
Mormons have no formal exorcism, though the devil is real.
"To deny the existence of Satan and the reality of his evil power and influence," said LDS apostle David B. Haight in 1973, "is as foolish as ignoring the existence of electricity."