I could add that people suffering from porphyria were sometimes in the old days thought to be vampires due to several symptoms often associated with these creatures. One of these is a sort of "allergy" to sunlight which causes rash and pustules to appear on the skin if you are outside during daylight. Also, the fact that porphyria is caused by a failure of the body when creating heme (the thing in your blood that carries oxygen and gives blood its colour) also supported the common belief that vampires need to suck blood. (and that they could that way make up for the lost heme in blood)
