(from my laboratory instructor)

Most interesting lesson in Chem261: Vampires do exist! They are people who suffer from Porphyria - a genetic deffect wherein there is an accumulation of Uroporphyrinogen I which stains the urine red, causes the teeth to fluoresce in UV light and the skin to be sensitive to sunlight. They are anemic (due to lack of Heme) thus, they crave for blood! :D