vampires
Who doesn't love a good vampire story? My inspirations for vampire goodness come from Bram Stoker's Dracula, the 20+ year old Castlevania game series, various spiritual woo-woo ideas, and my own imagination. I've obviously spent too much time thinking about genetics or something. :p
how it works
In my writing, vampirism is something of a spiritual virus. Okay, let me explain.
The vampire part of a victim is a low-grade, weak demon. These demons exist (un)naturally in the environment and tend to gather around people who are in physical or emotional pain. Since they are weak, they can't do anything on their own, and usually nothing happens.
However! When a person is bitten by a vampire, it's not just a simple sucking of blood. It's also an injection of sorts - the vampire's saliva is loaded with something like a virus that infects the victim's blood. Since it's physically impossible for a vampire to drink even half of the blood in a human's body (look it up - amount of blood in an adult human vs. stomach size), most of that person's blood stays with them after death, and the virus multiplies like any good disease should.
The virus goes around, rewriting the person's DNA (just like a real virus), and changes the body so it is suitable for hosting a demon. One of the gathered demons manages to beat out the others and wiggle its way in, taking residence in the area surrounding the person's heart. Once established, the demon starts using the human's infected blood as fuel for making more drastic changes. (More to come as I'm actively working this out for the next bit of Falling Down.)
The vampire demon bonds with the human soul, anchoring it to a corpse that it would otherwise flee. The demon is normally content to allow the human to be in control; after all, humans are intelligent creatures that are pretty good at taking care of themselves. The demon hangs out in the subconscious and emerges only when the vampire is in danger - from himself or others! - to take control of the body and remove the threat. So if a vampire refuses to eat, the demon will allow this for a while, then pop out and go on a feeding frenzy.
All this change ends up taking three days. Coincidence, I'm sure. :) The new vampire awakens, nearly bloodless and terribly hungry. If there's someone powerful available to take care of him, make sure he's fed regularly, and explain what has happened, then he'll be fine. If not... well, the stress is a bit much for most people, and we end up with a mindless, hungry beast.