By the time he had offered himself as a donor, however, his opinions had taken a U-turn. Many real-life vampires have no belief in the paranormal and have little more than a passing knowledge of True Blood or Dracula; nor do they appear to have any psychiatric issues. Instead, they claim to suffer from a strange medical condition – fatigue, headaches, and excruciating stomach pain – which, they believe, can only be treated by feeding on another human’s blood. “There are thousands of people doing this in just the US alone, and I don’t think it’s a coincidence, and I don’t think it’s a fad,” says Browning.  Their symptoms and behaviour are a genuine mystery.

It was not always this way; across history, we can find cases where human blood was considered a bona-fide medical cure. At the end of the 15th Century, for instance, Pope Innocent VIII’s physician allegedly bled three young men to death and fed their blood (still warm) to his dying master, with the hope that it might pass on their youthful vitality.

Later on, it was used to treat epilepsy; the afflicted were encouraged to gather around the gallows and collect the warm blood dripping from recently executed criminals. “Blood was a medium between the physical and spiritual,” explains Richard Sugg at the University of Durham, who recently wrote a book on “corpse medicine” and who is currently writing a volume on vampirism. By drinking the blood of a healthy young man, he says, you were imbibing his spirit and curing whatever afflicted your soul. These treatments only fell out of favour following the Enlightenment, and the onset of a more general sense of prudery that took hold in the 18th and 19th Centuries.

http://www.bbc.com/future/story/20151021-the-people-who-drink-human-blood