What was the king's illness?

Anybody want to speculate what the king's illness was specifically? I had assumed from the beginning that he was suffering a sexually transmitted disease consequent to his sex addiction. At the end on his deathbed he looked terrible, all black and oozing sores, sweating profusely, shaking uncontrollably while the his obsessed young queen wife, Joan, hugs and kisses him. Gross.
But wait...earlier in the movie, as the first symptoms of his illness appear almost imperceptibly, you see him always thirsty and drinking water. At this time in Europe's history, the wealthy citizens, aristocrats and the nobility drank wine instead of water. But you see the king/duke of Flanders drinking glass after glass of water. Somewhere I seemed to recall there are some illnesses among which one symptom is thirst.
Anyone want to chance a guess?