Porphyria is a disease that affects part of the haemoglobin in the blood. It causes the gums to recede, the skin to flake off when exposed to sunlight and can be treated by administering blood intravenously. Unsurprisingly, then, it is thought by some to have contributed to the vampire legend. Although there isn't any evidence to suggest that ingesting large amounts of blood would alleviate any of the symptoms of porphyria, some of the earliest depictions of vampires don't go in for that kind of behaviour anyway. The whole exposed-fangs-can't-go-out-in-the-sun thing would probably be enough to get a few imaginations whirring in an age of magic and superstition. There are even some people who think that members of various royal families suffered from porphyria, but this might be the cart leading the horse, springing up from the various the-royals-are-vampires/aliens/lizards conspiracy theories that float around the slightly crazier strata of society. Scientists aren't 100% what causes it, they think it's something to do with faulty DNA. Luckily, sufferers don't actually have to consume the life-force of the innocent to stay alive and it can be controlled to a certain extent with drugs and diet.
