h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

In medieval times when entire communities would be wiped out by disease/plague, often the first person to die would be blamed. The belief was that the dead person's spirit would rise out of the grave (not the physical body) and drain the life-force from their victims. The villagers would start digging up bodies, removing heads and/or vital organs, filling mouths with garlic/onions, burning entire bodies, etc. until the plague ran out of steam.

It wasn't until some time later that the European vampire was believed to physically rise from the grave and drink blood.

The paranoia surrounding plague victims was not simply confined to their spirits rising. There are accounts even in the British Isles of walking corpses shambling about under their own volition and passing on the fatal sickness to still more victims. Some researchers hold that this walking corpse myth later mixed with the far earlier tales of succubai and other creatures which date back to the biblical figure of Lillith (the first wife of Adam, cast out of Eden for her bold sexuality and challenge to male authority) and fed upon the sexual energies of their victims. Thus we have the modern vampire who spreads his "plague" from one victim to another, seducing them with his powerful sexuality...All of which of course neatly links to the modern paranoia of sexually transmitted diseases and moral degeneration, I mean just look at Blade, you have porn stars playing vampires, undead drug-dealers and nightclub owners. It all boils down to whatever the current overriding fear in society is at the time.