Horror films deal with the viewer's nightmares, hidden worst fears, revulsions and terror of the unknown. Although a good deal of it is about the supernatural, if some films contain a plot about morbidity, serial killers, a disease/virus outbreak and surrealism, they may be termed "horror."
Plots written within the horror genre often involve the intrusion of an evil force, event, or personage, commonly of supernatural origin, into the everyday world. Themes or elements often prevalent in typical horror films include ghosts, torture, gore, werewolves, ancient curses, satanism, demons, vicious animals, vampires, cannibals, haunted houses, zombies and masked serial killers.