A bit of research - Allergic to the sun

In "Racing the Dark", the world is ruled by vampires. While my vampires don't follow every rule the myths lay out for us, they do follow the "burn in the sun" bullet point.

Part-way through the story, Darin comments that, "It is derived from a human genetic disorder." That disorder would be Xeroderma Pigmentosum, and the short explanation of it is that the person's body can't repair the damage caused by UV light.

The MedicinePlus Medical Encyclopaedia defines it as "a rare condition passed down through families in which the skin and tissue covering the eye are extremely sensitive to ultraviolet light."