Vampires page 7
It has also been hypothesized that vampire legends were influenced by individuals being buried alive due to primitive medical knowledge. In some cases in which people reported sounds emanating from a specific coffin, it was later dug up and fingernail marks were discovered on the inside from the victim trying to escape. In other cases the person would hit their heads, noses or faces and it would appear that they had been "feeding." A problem with this theory is the question of how people presumably buried alive managed to stay alive for any extended period without food, water or fresh air. An alternate explanation for noise is the bubbling of escaping gases from natural decomposition of bodies. Another likely cause of disordered tombs is grave robbing.
Folkloric vampirism has been associated with a series of deaths due to unidentifiable or mysterious illnesses, usually within the same family or the same small community. The epidemic allusion is obvious in the classical cases of Peter Plogojowitz and Arnold Paole, and even more so in the case of Mercy Brown and in the vampire beliefs of New England generally, where a specific disease, tuberculosis, was associated with outbreaks of vampirism. As with the pneumonic form of bubonic plague, it was associated with breakdown of lung tissue which would cause blood to appear at the lips.
In 1985 biochemist David Dolphin proposed a link between the rare blood disorder porphyria and vampire folklore. Noting that the condition is treated by intravenous haem, he suggested that the consumption of large amounts of blood may result in haem being transported somehow across the stomach wall and into the bloodstream. Thus vampires were merely sufferers of porphyria seeking to replace haem and alleviate their symptoms. The theory has been rebuffed medically as suggestions that porphyria sufferers crave the haem in human blood, or that the consumption of blood might ease the symptoms of porphyria, are based on a misunderstanding of the disease. Furthermore, Dolphin was noted to have confused fictional (bloodsucking) vampires with those of folklore, many of whom were not noted to drink blood. Similarly, a parallel is made between sensitivity to sunlight by sufferers, yet this was associated with fictional and not folkloric vampires. In any case, Dolphin did not go on to publish his work more widely. Despite being dismissed by experts, the link gained media attention and entered popular modern folklore.