In 1964, in an article in the Proceedings of the Royal Society  of Medicine, L. Illis proposed that porphyria might be an explanation for werewolf legends. More than 20 years later, in 1985, David Dolphin presented a paper at a meeting of the American Association for the Advancement of Science, proposing that porphyria might be also an explanation for vampire legends.   

Porphyria is actually a group of diseases, all pertaining to the metabolism of porphyrin rings that, along with iron, are responsible for the oxygen-carrying properties of emoglobin, the red ingredient in blood. Porphyria is a very rare genetic disorder and is in no way contagious.

It may have developed among the European nobility due to interbreeding. One of the varieties of porphyria called congenital erythropoietic porphyria, is characterized by yellow and hairy skin that is extremely sensitive to light, forcing the sufferers to go out only at night or risk tissue damage. 

Ulcers may also cause their hands to become deformed and paw-like and mutilate the nose, ears, eyelids, and fingers. Even their behaviour becomes erratic and red pigments appear in their teeth and urine. In addition, some kinds of porphyria are associated with epilepsy.