Inevitably, superstition had a big part to play in the 'discovery' of the vampire. Graves were dug up and because people didn't understand the stages of decomposition, they assumed that the blood rushing to the deceased's face was because they had been feeding all night and that they were sitting up because they were still alive. This was their evidence for vampires, but they needed an explanation for how anyone could become a vampire in the first place. It's universally known that vampires create other vampires by biting the neck (or other body part) of a victim in order to change them. But what's the origin story? There are many different theories but the main one is that Satan created a vampire to terrorise humans on Earth, just for the sheer fun of it: a demonic creation sent from Hell to do the devil's work. Also, if someone was said to have committed suicide, that would turn them into a vampire because they are voluntarily giving up their soul and because it's a harmful and self destructive act, their soul goes to the Devil who turns them into vampires. But then again jumping over an unburied body as a nun, letting a cat jump on your body before it was buried or being born with teeth were all signs that you were also going to be turned into a vampire.
