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Vlad the Impaler had a skin disease, which was only kept in check by him having a munch on people.
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Actually Vlad was just a nut-job. But you're right in that many of the vampire legends were based on porphyria, a generic disease in which sufferers don't produce heme, a component of hemoglobin. This makes their skin sensetive so they can't go outside in daylight. Also the only way to treat it in the middle ages was to drink blood.
All information sourced from The Straight Dope
on Dracula (plenty of gory stories on Vlad the Impaler's wacky antics in there)