The link below takes you to a paper by Costas J. Efthimiou and Sohang Gandhi that describes why various monsters don't exist, based on scientific reasoning. It's very tongue-in-cheek and makes for interesting reading.
Direct link to PDF: http://www.arxiv.org/PS_cache/physics/pdf/0608/0608059v1.pdf
Anyone who has seen John Carpenter’s “Vampires” or the movie “Blade” or any of the host of other vampire films is already quite familiar with how the legend goes. The vampires need to feed on human blood. After one has stuck his fangs into your neck and sucked you dry, you turn into a Vampire yourself and carry on the blood sucking legacy. The fact of the matter is, if vampires truly feed with even a tiny fraction of the frequency that they are depicted to in the movies and folklore, then the human race would have been wiped out quite quickly after the first vampire appeared.
I have recently spoken to Costas and I'm very pleased to announce that he's given permission for me to use this paper in the follow-up to Rebirth...