Lt. Elgin: Spivy! You bash that prisoner one more time, I'll put those shackles on you.
Major Fambrough: Sir knight? I've just pissed in my pants... and nobody can do anything about it.
John Dunbar: The strangeness of this life cannot be measured: in trying to produce my own death, I was elevated to the status of a living hero.
John Dunbar: Dunbar, not Dumb Bear.
John Dunbar: I am Lieutenant John J. Dunbar and this is my post.
John Dunbar: Many times I'd felt alone, but until this afternoon I'd never felt completely lonely.
John Dunbar: Guns would make one warrior like two.
John Dunbar: How come we haven't seen any buffalo?
Timmons: Can't figure the stinking buffalo. Sometimes you don't see them for days, and sometimes they're out there as thick as curls on a whore.
John Dunbar: What about Indians?
Timmons: Indians? Goddamn Indians you'd just as soon not see, unless the bastards are dead. They're nothing but thieves and beggars.
Answer: He was mentally disturbed and was depressed about being assigned to a "dead end" post with no chance at advancement. Dunbar, the hero, choosing to be assigned to the frontier, just pushed the poor soul over the edge.
Mark English
In a word, the disease syphilis. The urinary tract problems and the Insanity are possible side effects.
What are you basing this on? What in the movie indicates that he has syphilis?