From Jennifer Ouellette's The Physics of the Buffyverse
Buffy battles a Turok-han in "Bring On the Night"
"Yet there are aspects of vampire lore that resemble the symptoms of real diseases. Most notably, porphyria is a hereditary disease in which the body doesn't produce sufficient heme, an iron-rich pigment in the blood. Those who suffer from certain types of porphyria are highly sensitive to sunlight and may have reddish mouths, like the ancient Master vampire with "fruit-punch mouth" who was introduced in Season 1 of Buffy. Some historians suspect that a common folk rememdy for porphyria may have been to drink fresh blood, but if so, those efforts were wasted. The chemical enzymes in the blood that sufferers require can't survive the digestive process; they must go straight into the bloodstream via blood transfusions or injections.
Over time the most severe (and rarest) forms of porphyria can cause blistering, scarring, and thickening of the skin, and in extreme cases can lead to disfigurement. The lips and gums may become so taut that the teeth protrude like fangs, giving the sufferer an appearance strikingly similar to the Nosferatu of early horror films, or the Buffyverse's Turok-han, an ancient race of übervamps. In fact, the writers of Buffy are on record as saying that they originally conceived of vampirism as a progressive disease, and the Master's appearance reflected that. But the similiarities between vampirism and the symptoms of porphyria appear to be entirely coincidental. There have been only two hundred or so documented cases of the most extreme forms of porphyria, hardly enough to inspire the plethora of vampire legends around the world, and many of the cited vampiric attributes didn't appear in folklore until the nineteenth century.
We can find clues to explain vampires' extremem sensitivity to sunlight not just in the enzyme deficiencies that characterize porphyria and similar disorders, but also by looking at how the sun's rays cause human skin to tan and burn. The sun emits three forms of light: infrared light (heat), visible light, and ultraviolet (UV) light. It is the latter that is responsible for skin damage: prolonged exposure can damage and kill skin cells, which then release chemicals that activate the body's pain receptors. The reddening of sunburned skin is the result of increased blood flow to the damaged areas in order to remove the dead cells. The energy from UV light also stimulates the production of a pigment known as melanin, which causes the skin to darken, or tan. Melanin actually absorbs UV radiation in sunlight, protecting skin cells from further damage."
I realize by posting this and admitting that this is the book at the top of the heap on my bedside table (currently covering Claudia Emerson's Late Wife) that I'm tipping my nerd hand down—way down. But for someone who struggled to understand any science beyond chemistry, this book is probably the easiest (and most fun) course in science I've ever gotten to take.
There are a dearth of scholarly books exploring various aspects of Buffy—philosophy, philology, ontology, rhetoric, feminism, space, etc. They're worth a read if you enjoy the show.