A metabolic disorder - or rather a group of disorders - that result from improper breakdown of haem. The same pigment that makes blood red causes problems if not dealt with completely by the body. Some sufferers have skin problems, especially after exposure to light which seems to sensitize the skin. More dangerous is an acute attack - vomiting, constipation and even mental disturbances like depression, hallucinations or schizophrenia. Attacks can be brought on by diet, certain pharmaceuticals and alcohol.

Interestingly, George The Third may have been a sufferer. His behaviour under the influence of severe attacks, combined with symptoms such as the colour of his stools suggests porphyria. Since certain types of porphyria are inherited, the disease can be traced through the royal family (in much the same way as haemophillia was).

Culled from various internet sources, and a play I saw once.