Well, it turns out that the symptoms of vampirism are very, very close to symptoms of the disease furious rabies (which we're sure will star Vin Diesel some decades in the future). It's a neurological disease that attacks the brain and is incurable unless you have a very unpleasant set of shots to the belly if you get bit by an infected animal.
Without those shots, its symptoms include a laundry list of vampire characteristics: an aversion to strong stimuli, including bright light and strong smells (read: garlic); hydrophobia, so you'd be thirsty as all get-out but unable to overcome your fear of water (read: holy water); and a fear of breezes and mirrors (read: Edward Cullen).
Even weirder, the disease makes it really hard to swallow, so the victim starts foaming at the mouth and hissing, because again, why would a vampire hiss? We want vampires to be scary, and we call it a hissy-fit for a reason. Of course, furious rabies' biggest kicker is that the victim quickly gets crazy aggressive and wants to bite everything that moves, basically acting like any rabid animal would.
European vampire hysteria often coincided with massive outbreaks of furious rabies, like the one in Hungary in the 1730s. And the European animals that spread rabies -- wolves, dogs, bats -- are all associated with vampirism. Combine that with the fact that fingernails and teeth appear to grow after death (thanks to dehydration), and decomposing entrails take on a dark red color that looks like horror-movie blood (thanks to the ever-presence of death that we can only distract through dick-jokes), and you've got the makings of an excellent, scary myth.