The vampire is the stuff of the great horror stories. However, a recent textbook by geneticist John Jenkins suggests that Bram Stoker’s legend of the vampire, Dracula, may have had its basis in a very real disease that prevents some people from getting rid of old red blood cells. In healthy people, red blood cells live only a short time and then are broken down by the body, which uses the chemical components of old cells to make new ones. But in a hereditary form of a disease known as “porphyria,” a missing enzyme prevents the body from breaking down hemoglobin in old red blood cells. What happens instead is that the hemoglobin is only partially broken down, the result being some very odd symptoms.