In the Middle Ages, in Western culture, it was seriously believed that madness was caused by demonic possession. Satan, or devils, or demons, entered into the person and took possession of the person's soul. People would then try to cure the person by means of religious rituals called "exorcism" or "casting out devils." The New Testament, the fundamental religious book of Christianity, refers to Jesus curing people by "casting out devils."
Because it is mentioned in the New Testament, I am sure there are fundamentalist Christians in the United States today who believe in demonic possession and exorcism. I don't believe in it, and nobody I know believes in it.
However, the idea survives in common sayings or idioms. When I was disobedient as a child, my mother would say to me "I don't know what's gotten into you." It was just a phrase, she wasn't seriously thinking about demonic possession! People will say someone "has the devil in his eyes" or "her eyes" or "has an impish spirit" meaning they are planning mischief.
"The Exorcist" was a hit film of the 1970s. Was the film seriously suggesting that exorcism was real? Like "The Da Vinci Code," I think it was merely a horror story of the supernatural, which only played with the theme.