Sanguinarians are a type of real, human vampire who drink blood, generally from willing human donors. Most report a decline in health if they refuse or are unable to feed in this fashion. The two primary theories behind this are that it is a nutritional deficiency that can only be met by consumption of living blood, or that it is a life-energy deficiency, and blood is a tie to, or contains large amounts of, this energy.
Sanguinarians are, in fact, quite alive, and are not to be confused with supposedly fictional, undead vampires.
- Therian.Wikia has more info on these real-life Vampires.