In his article: http://www.straightdope.com/columns/990507.html
Cecil makes a persuasive case that porphyria can’t be an explanation for vampire legends. He also notes that porphyria had been proposed as an explanation for legends about werewolves.
This is interesting. I had always thought lupus was the classic explanation for werewolf legends. Is there anything to this, or am I just getting confused by etymology?