at http://www.livescience.com/othernews/061025_vampire_debunk.html we find...
A researcher has come up with some simple math that sucks the life out of the vampire myth, proving that these highly popular creatures can't exist.
University of Central Florida physics professor Costas Efthimiou's work debunks pseudoscientific ideas, such as vampires and zombies, in an attempt to enhance public literacy. Not only does the public believe in such topics, but the percentages are at dangerously high level, Efthimiou told LiveScience.
Legend has it that vampires feed on human blood and once bitten a person turns into a vampire and starts feasting on the blood of others.
Efthimiou's debunking logic: On Jan 1, 1600, the human population was 536,870,911. If the first vampire came into existence that day and bit one person a month, there would have been two vampires by Feb. 1, 1600. A month later there would have been four, and so on. In just two-and-a-half years the original human population would all have become vampires with nobody left to feed on.
but this is bad math. Suppose every time the Vampire population reaches 20 or so, all but one of them are killed by angry villagers? Then vampires can exist.
Of course, this doesn't tell us where that first one came from on Jan 1, 1600....