Another interesting theory I read in relation to Vampirism is that it may have been caused by Porphyria. Not only do people with some types of porphyria have photosensitivity and disfiguration of the face, they are also shown to have exaggerated reactions to certain chemicals in garlic. In addition, is it implausible to suppose that they'd go out in search of blood to replace the blood that their body could not manufacture properly?