Porphyria! The Mystery of the Vampire Myth Explained
TPG CLASSIC - REPOST from 31st October 2008.
Teeth lengthened by receding gums glow in the shaded valleys of the Carpathian basin. The genetic milieu of the Visigoths, Huns, Carpians, and Slavic peoples have swirled over centuries as granite crags pushed populations into shallow gene pools. A ghoulish thing of legend emerged from Transylvania and has since soaked literature and pop-culture with a reddish froth. Science and medicine have given us reason not to fear for our necks in a story that is truly stranger than fiction...
The glowing teeth mentioned earlier occur because of an accumulation of light sensitive

Mal-formed porphyrin molecules accumulate in affected people's skin and teeth. When activated by light these molecules activate oxygen molecules into destructive radicals that can wreak havoc on surrounding tissue. When exposed to light individuals appear to be burned with caustic lesions left on their skin. Enter: Nosferatu's pale countenance shrieking in terror as the sun rises.

In the genetically secluded valleys of Transylvania it is not hard to imagine disorders like these occurring more than expected in a diverse population. Before the renaissance and the spread of the scientific revolution a vampire myth may have grown from accounts of these individuals with their glowing long teeth, aversion to sun and garlic, and maybe even sucking blood in the night!