Note to Twelfth Night, 1.3.18, "Aguecheek"

An ague (AY-gyoo) is a condition of alternating chills and fever caused by disease, particularly malaria. Sir Andrew Aguecheek doesn't have the ague, he just looks like he has it, and—as in Julius Ceasar—Shakespeare is probably thinking of the ague as a condition that makes one pale and thin. If this is true, Sir Toby Belch and Sir Andrew Aguecheek probably look funny together; Sir Toby is round and ruddy, and Sir Andrew is the opposite.