Probably asked before...

What.

Actually, his malady is never stated, but he has lost the will to fight for his life.

It was never made exactly clear what exactly caused the King's death, but his will to live was definitely compromised when he was shamed by Anna when she prevented him from beating Tuptim and once he realized that his son Chululongkorn was able to handle being the next King, he stopped fighting and passed away quietly. It's so sad.

The King died because he felt that he had lost for ever Anna's respect. It must have been very painful to hear her say,"You have no heart! You never loved anyone and you'll never will!" These words are said just a few moments after the King expressed his feelings to Anna for the first time in such an extrovert way. Although he couldn't understand why she disagreed with polygamy, for a while he became fascinated himself by her picture of "a young girl in her first dance..." without even realizing it. When Anna rose herself against him, the King didn't manage to overcome the fact that the only woman he really respected still considered him a barbarian. Notice that after throwing away the whip, the King looked at the guards, the lowest in rank. They turn their faces as if they were ashamed for him, because he demonstrated weakness and he wasn't anymore the monarch with the absolute principles. Anna had managed to make him doubt and search for other options. The King's challenge was to conciliate his role as an absolute monarch and as a thinking progressive leader. Disappointed when he failed to do so, he lost his spirit and let himself die, opening the way for his son to achieve this double goal with Anna's help. The moment he dies the music from "Something Wonderful" can be heard in the background to remind that the King deserved respect because at least he TRIED, and he tried very hard.

I believe that this musical owes much of its fascination to the fact that each of us can give a different explanation. I find particularly clever and moving that the King died silently, discreetly, while Chulalongkorn made his proclamations. The old died while the new was born.

Memento vivere (Remember to live)

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According to what Lady Thiang tells Anna, the king went into a severe depression after the episode with Tuptim, and wouldn't see anyone or take food or drink. So one would assume that the king dies of malnutrition.