The Horror fiction genre requires that the reader engages in a conspiracy that agrees to suspend the rules of everyday. Readers must invest strong psychological belief in the literary worlds that are presented.
Horror is an experience of safety and conservatism. Readers are allowed to risk it all from the pleasurable nook of their protected and rule-governed psychological "home" through contrast with the terrifying and the chaotic.
As may naturally be expected of a form so closely connected with primal emotion, the horror-tale is as old as both human thought and speech themselves. Let's face it: Horror has been around as long as man and his fear of the dark.
The inhabitants of the horror fiction genre are also as old as storytelling itself. Most familiar are ghosts, demons, vampires, zombies, etc, that seemingly inhabit a twilight zone that occasionally seeps into our own reality, especially around times like Halloween. The thin membrane that separates the living from the dead opens fleetingly and something, usually evil, slips through.
(Information obtained at Find Me an Author)