From All's Well That Ends Well, Act I, scene 1:
BERTRAM
What is it, my good lord, the king languishes of?
LAFEU
A fistula, my lord.
BERTRAM
I heard not of it before.
LAFEU
I would it were not notorious.
According to this article abstract:
Textual and contextual evidence suggests that the French king's fistula, a central plot device in Shakespeare's play All's Well That Ends Well, is a fistula-in-ano. Anal fistula was known to the lay public in Shakespeare's time. In addition, Shakespeare may have known of the anal fistula treatise of John Arderne, an ancestor on Shakespeare's mother's side.