Everything you need to know about exorcisms (which isn't much).
Before the germ theory of disease came about, it was largely believed that illness was the handiwork of crafty demons. Only the morally corrupt and unrepentant were susceptible to demonic possession. Here's a classic and oh-so-common example (if you'll rewind the clock a few hundred years): the dirty slag your friend Bill slept with fifteen years ago unknowingly gave him syphilis. The infection took its long, symptomless course, spreading to his brain and quickly turning it into Swiss cheese. Bill now does strange things like eat his own fingernails and babble incoherently. Modern medicine would treat such a thing with antibiotics.
But demonic possession seemed the only reasonable explanation in the era of ignorance and pervasive church presence. Thus, your friend Bill would've had what the Catholic Church calls an "exorcism." During this process, Bill would be splashed with scalding holy water, deprived of food, and have Biblical passages yelled at him for days. Bill would die of trauma, and the demon would be vanquished. The clergymen would dance and boast about the power of Christ and collect their fee. Bill's neighbors would take notice and be more devout and pious as a result. Oh, and Bill would be dead.
Perhaps a bit too much.
While they're not as popular as they used to be, exorcisms still occur. The Catholic Church and some Protestant sects still recognize demonic possession as legitimate and allow exorcisms. Two historical figures who have undergone exorcisms are Salvador Dali and Mother Teresa. Mother Teresa believed she was succumbing to evil spirits when she found herself believing that condoms weren't worse than AIDS.
Mother Teresa, around the time of her demonic possession. Note the monochromism.
The most well-known exorcism is that of Anneliese Michel. Anneliese, a German Catholic, became convinced that her epilepsy and schizophrenia were the product of demonic possession after much insinuation by her close friend and an exorcist who hadn't had any customers as of late. More than ten months of semi-starvation and bizarre rituals later, she died in her sleep. The shitty fearmonger film The Exorcism of Emily Rose is loosely based on this.
You no doubt want to read about The Exorcist, The Last Exorcism, and the plethora of other movies and media relating to, well, exorcism. While this article won't address these films per se, we will say that Hollywood has bought and sold the scary idea of exorcisms and demon spirits to millions of viewers.
While fantasy and horror are often effective as a "what if" (even ones which claim themselves to be "based on a true story", or "based on actual events" for added effect), movies about exorcisms are notably esoteric and moronic because there are people who actually believe that demonic possession is real. You've seen them. The people on TV being smacked in the face by some overweight, balding preacher who is screaming, "GET OUT DEMON! GET OUT!" Laughable to most, but real to some.
He knows a thing or two about exorcisms.
This is unlike vampire movies, movies with shape-shifting monsters, movies with winged creatures that eat people, movies with radioactive dinosaurs that terrorize Japan, and whatever the hell Cloverfield was. Hollywood has a head-up with exorcism movies because of two demographics they can easily count on to watch them, which are "people who actually believe in this crap" and "people who mistook it for an ironic Michael Moore documentary about physical fitness, then left." Everything is green in Hollywood.