Clive breaks it down:

Vampires can’t exist. Why? Because they’d quickly depopulate the earth. To prove it, the scientists do some calculations by picking a random year in history — 1600, specifically — and imagining what would happen if one person suddenly appeared on earth.

They assume, for the sake of argument, that a vampire needs to feed “only once a month”, and that in the course of feeding, the vampire turns its victim into another vampire. They note that the global population of humans was 536,870,911 in the year 1600.

Then the calculations begin. If a single vampire fed on a single human in the first month, this would create two vampires — and decrease the human population by one, leaving it at 536,870,911 - 1 = 536,870,910. In the second month, those two vampires would each feed, transforming two people into vampires — so you get four vampires and a human population of 536,870,911 - 3 = 536,870,908. So you can see where this is headed. The vampire population is increasing in a geometric progression, and the population of humans is similarly decreasing — and at that rate, the authors calculate, the entire human population would be transformed into vampires in only 30 months. QED.

Of course this is bullshit because vampires don’t turn anyone they feed on into a vampire—and that’s the key assumption that gets the calculations going. So all you vampire believers, hope’s alive.

Via New Shelton.