In 1985, a Canadian scientist, David Dolphin, at a conference of the American Association for the Advancement of Science (AAAS) came up with a theory to explain Vampires but this explanation has been discredited as a vampire theory according to The Vampire Book: Encyclopedia of the Undead by J. Gordon Melton, a religious scholar, citing that it didn't live up to what a Vampire was, and stigmatized people who actually had the disease.
The theory is; people who were and are accused of being a Vampire suffered from the disease called Porphyria, the human bodies inability to produce heme, to carry oxygen in the blood. Over time different chemicals build up under the skin, which causes it to be sensitive to sun light. The whites of the eyes, teeth and hair develop a reddish tone. Painful open sores develop, hair grows wiry. All of these symptoms are legendary characteristics of the vampires of yore.
No wonder people in medieval times believed in Vampires.
People then, usually dug up the dead ones they suspected of being a vampire. Sometimes when a body is buried, during natural decomposition the body will appear bloated as if full, blood can still be a liquid for weeks after death and ooze out through the mouth, giving the appearance it had just drank someone's blood. If the grave is cool enough, the body may look as if it hasn't decomposed at all.
That was all the proof needed to confirm their beliefs. Vampires made a hell of a lot of sense when there were no other answers to questions about death and the plague and anything else they had no explanation for.
They would cut the head off and place it between the legs. Then pin the body down with a wooden stake driven through the heart, sprinkle it with holy water then cremate the whole body as they prayed.
It was said, this would stop a Vampire.