The late Susan Sontag wrote a book that decried the use of illness as metaphor, and argued that we should abandon the practice. If Shakespeare had followed her advice the plays would have been somewhat shorter, for medical metaphors are very frequently employed in them.

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 …