Legends and mythology have basis in fact, and in this case the facts aren’t far off. There are over 200 cases of subjects who blister and burn in direct sunlight and present with fangs. Some subjects turn furry, a la Wolfman. Blood injections seem to be the primary treatment.
Subjects suffering congenital erythropoietic porphyria have mutations that interfere with porphyrin-heme synthesis and experience a number of neurological and skin conditions. The disease is autosomal recessive, and 2 faulty alleles for coding the enzyme uroporphyrinogen III synthase cause subjects to accumulate biologically useless porphyrins (type 1 isomers). The result if sensitivity to light resulting in blisters, scarring, and skin-erosion.
In rare cases, subjects’ gumlines can recede to give the appearance of fangs. Their spleens become enlarged, and their bones become fragile and brittle. Teeth and urine darken. Subjects may then proceed to grow abnormal amounts of hair (hypertrichosis), a condition informally termed the “werewolf syndrome.”
Scientists are unsure of why alcohol, estrogen, drugs, and infection exacerbate the disease. Treatment includes intravenously administered hematin, pain medication, sedatives, blood removal, and a number of dietary supplements.
It’s interesting to note that Bram Stoker wrote Dracula (1897) within a decade that Dutch physician Barend Stokvis first described acute porphyria (1889). Bram Stoker–Barend Stokvis. Vampire book–vampire disease. Coincidence?! I think not. Queue conspiracy theory.