In A Nutshell
Porphyria is a hormone disease that causes sufferers to develop horrifying physical symptoms and light sensitivity. It is believed that severe cases of the little-understood disease gave rise to the legends of vampires and werewolves.
The Whole Bushel
Porphyria is a genetic disorder of porphyrins (organic compounds of which heme—red blood cell pigment—is the most common in humans). Of the two types of porphyria (one affecting the nervous system and one affecting the skin) the most horrifying is cutaneous porphyria (porphyria of the skin). The most common symptoms are blisters, necrosis (destruction of the flesh), swelling, redness, itching, and severe light sensitivity.
The term porphyria comes from the Greek word for purple because some sufferers develop purple-colored feces and urine when they are exposed to sunlight (due to excess porphyrins near the surface of the skin). This same exposure can also cause extensive hair growth on the forehead which could be—in part—the root of werewolf myths.
A person with porphyria experiences extreme pain and nausea which requires treatment with strong opium-based painkillers. Even a person with few physical symptoms can have excruciating pain. The physical symptoms of the disease and the need to shun light is almost certainly behind the various legends of vampires found all around the world.
The humiliation and pain caused by this disease is bad enough, but to make matters worse, sufferers of porphyria were frequently treated with electroconvulsive therapy due to a lack of understanding of the illness. Needless to say, it didn’t help.
In ancient history (before vampires were spoken of) blood consumption and other vampirish acts were attributed to demons. It wasn’t until Christianity began taking a hold around the world that the demonic tales began to morph into the walking-dead style of vampire that we are all more familiar with. This is perhaps reminiscent of the rising from the dead that is found in Biblical tales.