Why are vampires allergic to the sunlight?

Answer 1

The human body is capable of a few photosynthetic processes: the production of vitamin D, certain cancer controlling processes, the production of melania, ect. Sometime however something goes wrong with these processes and instead of making useful things, as they are suppose to, they can reduce organic oxygen and phosphorus to their most active forms which then attack the flesh directly. These are actual conditions known also in living people who are required to hide from direct sunlight. (See Wikipedia for such things as Xeroderma pigmentosum, Erythropoietic protoporphyria, ect.) (please note: we are NOT saying people with these aliments are vampires, only that vampires suffer from these ailments as well)

Becoming a vampire is a complex process (biologically) and things do sometimes go wrong in the creation of new vampires, one of the more frequent faults being the malajustment of these photo-processes making individuals photo-sensitive.

There is a range of sensitivity known. Count Dracula could walk about in full day-light (dressed in Victorian costume, which is pretty well full body coverage, save the face). The vampires in the twilight series merely glittered (which is just a little odd). In Innocent Blood they broiled. In 'Interview with a Vampire' they were burnned down to ash. Count Orlock was vapourized in a matter of seconds.