I was a huge Buffy the Vampire Slayer fan (still am) and sometimes on the newsgroups or other sites there have been speculation about the creation of vampires, and this is my pseudoscientific explanation.
(Okay, for some reason, when some people see the word 'pseudoscietific' they seem to think it means 'kind of scientific' and therefore - is real, legitimate science. But 'pseudo' means "false."
Pseudoscience is false science!)
Vampirism is caused by a virus, and for a lack of a better name, I call it the 'vampovirus'. These vampoviruses are very weak and are easily killed by the white blood cells we have coursing through our veins.
Most of the time a vampire sucks out enough blood to satisfy his hunger, which also kills the person, but any vampoviruses he leaves behind are easily killed by the white cells.
But what if he/she happens to suck out so much blood that there aren't enough white cells to kill the invading vampoviruses? Three of four days later the vampoviruses have reproduced enough of themselves that a new vampire is created (and that is why so many of them have to dig themselves out of their graves).
But - when vampires DO want to sire someone, they suck out most of the blood, then they let the person drink some of their blood in order to flood the body with vampoviruses (but not knowing about vampoviruses, they don't know why it works, just that it does).
And when they do, there are now more vampoviruses flooding the body than the white cells can kill. And in just a few hours -- another vampire.