Yes, I think that's it -- biting as a way of passing on an infection, springing from a period when infection itself was imperfectly understood. They surely must have seen the result of being bitten by a rabid dog. Magnify the dog into a wolf, and you have a dangerous night prowler whose bite leaves the victim, well, contaminated. And some bats do suck blood. I once read that periods of vampirism -- that is, belief in vampires -- coincided historically with outbreaks of cholera. Some cholera victims were thought dead when they were only in a coma and were buried alive. Somebody coming out of a coma and finding himself in a coffin would do his damnedest to fight his way out, and evidently enough of them succeeded to fuel the legend of the dead-but-not-dead. (I don't know when the term "undead" came into use, but I suspect that's a modern addition.) So I guess if Dracula hadn't brushed his teeth for a while, he could very well pass along an infection.