A horror film is a film that seeks to elicit fear for entertainment purposes. Initially inspired by literature from authors like Edgar Allan Poe, Bram Stoker, and Mary Shelley, horror has existed as a film genre for more than a century. The macabre and the supernatural are frequent themes. Horror may also overlap with the fantasy, supernatural fiction, and thriller genres. Horror films often aim to evoke viewers' nightmares, fears, revulsions and terror of the unknown. Plots within the horror genre often involve the intrusion of an evil force, event, or personage into the everyday world. Prevalent elements include ghosts, extraterrestrials, vampires, werewolves, demons, Satanism, evil clowns, gore, torture, vicious animals, evil witches, monsters, giant monsters, zombies, cannibalism, psychopaths, natural, ecological or man-made disasters, and serial killers. Some sub-genres of horror film include low-budget horror, action horror, comedy horror, body horror, disaster horror, found footage, holiday horror, horror drama, psychological horror, science fiction horror, slasher, supernatural horror, gothic horror, natural horror, zombie horror, first-person horror, and teen horror.