"Throughout history the legend of the vampire has been used to "explain" other natural phenomena that primitive people who lacked scientific knowledge could not otherwise explain. Possibly the most astonishing belief which people associated vampires with was the Black Death during the Middle Ages in Europe. The Black Death, as we now know, was actually Bubonic plague spread by fleas and rats. The plague (which came from the East, not unlike the vampire) may have killed as much as a third of the population of Europe in the 1300s. Some people of the day, however, associated the multitude of deaths with vampires. Somehow they believed that the deaths were the workings of these monsters; perhaps the vampires spread plague, they may have thought. In some cases people believed a deceased relative returned as a vampire and killed a victim (who actually died of the plague). Alternately, it was believed a dead enemy could return and kill someone turning the victim into a vampire as well. Many graves were dug up and the bodies of suspected vampires mutilated to "kill" the vampire. Idiotic methods were used to "locate" the graves of vampires. For example, a virgin was placed naked on a horse, and the horse was paraded through a graveyard. If the horse (which was apparently more intelligent than the people) decided not to walk over a certain burial site, this was assumed to be the grave of a vampire. The body was immediately exhumed and mutilated to "kill" the vampire and, yes, thereby stop the plague which was devastating the region. Some of the most foolish vampire beliefs involved the methods used for killing vampires or stopping the spread of vampirism. It is important to remember, however, that while these beliefs seem absurd today, in an age when ignorance ruled unchecked, desperate people became susceptible to the power of superstitions. Corpses were sometimes buried face-down. If the corpse became a vampire it would actually dig deeper into the ground in an attempt to escape the grave, if it was facing the wrong direction -- or so it was thought. Wooden stakes were sometimes planted in the ground above the grave, so if the body rose it would stab itself on the stake -- hopefully through the heart."
http://www.parascope.com/en/articles/vampires04.htm