Today I watched an episode of the "CSI: Las Vegas" series. It was about a woman who used her dog to kill runners in the area she lived. At first our "anti-hero" Gill Grissom thought that she was doing it to sell their organs... Turned out she made "milkshakes" with them!
You may wonder "What does that got to do with the por...err...something you wrote on the title?"...
Well, Porphyria is an hereditary kind of digestive disorder (which she had...), more acurately called abhnormalitie, that is sometimes associated with vampirism... The symptons are awesomely similar! So she had to absorve human proteins to "remain a human" (or else... she would look like a monster).
From the site of the "National Digestive Diseases Information Clearinghouse (NDDIC)":
"Porphyria is a group of different disorders caused by abnormalities in the chemical steps leading to the production of heme, a substance that is important in the body. [...] Multiple enzymes are needed for the body to produce heme. If any one of the enzymes is abnormal, the process cannot continue and the intermediate products, porphyrin or its precursors, may build up and be excreted in the urine and stool."
The disease has different groups separated based on the effects it has in the body. So, as stated in the NDDIC Site:
"The cutaneous porphyrias affect the skin." (people can't stay exposed to the sun or else they'll get blisters and such) and "The acute porphyrias affect the nervous system" (which carries along all the nasty pains we have when we are in an nervous breakdown: pains in the chest and several muscles, paralysis, vomiting, constipation, personality changes, mental disorders, etc...).
It's an hereditary disease than can be passed by only one of the progenitors (autosomal dominant gene) or both (autosomal recessive gene).
As for the whys... The same site says the following:
"Attacks of porphyria can develop over hours or days and last for days or weeks. Porphyria can be triggered by drugs (barbiturates, tranquilizers, birth control pills, sedatives), chemicals, fasting, smoking, drinking alcohol, infections, emotional and physical stress, menstrual hormones, and exposure to the sun.".
The great variety of symptoms an the fact that they are common to other variety of diseases make it's diagnosis difficult and very complex.
Porphyria can be treated directly with heme, by medicines or drawing blood.