The reason vampires are associated with blood drinking is that "vampire" means "blood sucker" and porphyria was a common illness. Porphyria is a disease that causes irrational behavior, a phobia of light, paleness, and a desire to ingest blood. A common cure in the 1800s was a cup of cow's blood from a butcher to avoid attacking people. The folklore of vampires coming from the grave to drink blood derives from when a corpse is exhumed, and due to gasses built up in the body pushes blood out of the orifices, usually the nose and mouth. With an outbreak of a disease that causes erratic behavior, and corpses with blood around the mouth people, came to the assumption that the dead had risen to feed.
Vampires and werewolves
Porphyria has been suggested as an explanation for the origin of vampire and werewolf legends, based upon certain perceived similarities between the condition and the folklore.
In January 1964, L. Illis' 1963 paper, "On Porphyria and the Aetiology of Werwolves", was published in Proceedings of the Royal Society of Medicine. Later, Nancy Garden argued for a connection between porphyria and the vampire belief in her 1973 book, Vampires. In 1985, biochemist David Dolphin's paper for the American Association for the Advancement of Science, "Porphyria, Vampires, and Werewolves: The Aetiology of European Metamorphosis Legends", gained widespread media coverage, thus popularizing the idea. http://en.m.wikipedia.org/wiki/Porphyria
Origins of Belief
While the majority of vampire scholars focus on the cultural roots of vampire lore, some historians have looked to physical origins. There is no scientific evidence of actual vampires, but there are a number of real medical conditions that might result in vampiric behavior or appearance.
One of the most interesting "vampire diseases" is porphyria. Porphyria is a rare disease characterized by irregularities in production of heme, an iron-rich pigment in blood. People with the more severe forms of porphyria are highly sensitive to sunlight, experience severe abdominal pain and may suffer from acute delirium. One possible treatment for porphyria in the past might have been to drink blood, to correct the imbalance in the body (though there's no clear evidence of this). Some porphyria sufferers do have reddish mouths and teeth, due to irregular production of the heme pigment. Porphyria is hereditary, so there may have been concentrations of sufferers in certain areas throughout history. http://science.howstuffworks.com/science-vs-myth/strange-creatures/vampire5.htm
