From Dalrymple’s British Medical Journal column (subscription required):
In Twelfth Night, the very name of one of the principal characters, Sir Andrew Aguecheek, is a medical reference. What would you expect of a man who suffered chronically from the ague? That he would be lean, sallow, and weak, without much in the way of willpower: precisely the character of Sir Andrew.
There is a medical metaphor in only the nineteenth line of the play. The Duke of Illyria, Orsino, describes the effect upon him of the Countess Olivia, with whom he is in love but who has retired into mourning for a brother who has just died of a cause that we never learn: “Methought she purged the air of pestilence.” Olivia falls in love instead with Viola, disguised as a boy, who is sent to her by the Duke to woo on his behalf: “How now? / Even so quickly may one catch the plague?”
The plague, of course, is love, but (non-metaphorically) also a disease that scholars say exercised a profound and even determining effect upon Shakespeare’s literary career.