I saw an interesting essay linking rabies with vampirism.
Rabies itself was spread in eastern europe primarily through two animal vectors: bats and wolves. Hence, the association: someone comes in contact with these, the person becomes rabid, and is now a "vampire"
Symptoms of rabies itself bears striking parallels to vampirism.
People in latter stages of rabies start to have extremely hightened senses, light itself becomes painful to the eye, including the sun and mirrors. As well, strong scents become a deterrant, hence the garlic protection.
Rabies affects the brain as well, causing the sufferer to suffer psychosis and severe hydrophobia. They also seem to have a fixation with biting, which would be the means of transmission to family members.
People infected with rabies are also afflicted with insomnia, and are typically unable to sleep during the night and are up and roaming. Facial spasms cause the lips to jerk back and reveal teeth more prominently. Rabies victims also do not swallow their saliva, and vomit blood. The saliva is often tinged pink from regurgitated blood. If you see someone attack a person, and then later vomit blood, seems a logical conclusion that you'd then assume that they were drinking the blood?
Death from rabies (forget what typically causes death), often prevents the blood from coagulating/coagulates slowly. As well, Eastern Europe tends to be cold and wet, preserving bodies. The conditions are also ideal for a process which turns subcutaneous layers of fat into a wax-like substance. When the bodies were exumed, there would appear to be little decay and blood would still flow
Combine this with the fact that vampire legends themselves only became as prominent as they were in the 16th-17th century, which just HAPPENED to coincide with a major outbreak of rabies in Eastern Europe, and it would seem that rabies is explained.
To verify what I've said, do a google search of "rabies giving rise to vampire legends in Eastern Europe" and you should get some hits.
The parallels, in my mind, are FAR to similar, and in fact rabies explains pretty much all the archetypal vampire traits. If it looks like a duck, quacks like a duck...
Vlad the Impaler was NOT considered a vampire, btw. His name was taken by Bram Stokers, but before Bram there was no association with vampires.
Vlad the Impaler was a romanian (wallachian I think?) hero. He drove back the ottoman (i think) turks, and hurt them so badly and scared them so much with his man made "forests" that he was able to somehow preserve the orthodox christian state from the muslims. He wasn't a nice guy, he was extremely ruthless and thought nothing of impaling tens of thousands of his enemies on spikes, but he had a REASON to do it. He didn't do it for fun, he did it for the immense psychological impact it would have on his enemies.
He was no worse then the "enlightened" romans who would erect 'forests' of their own. Impaling, while more grisly, is not all that much worse then crucifixion.