For whiles this honest fool
Plies Desdemona to repair his fortune,
And she for him pleads strongly to the Moor,
I’ll pour this pestilence into his ear,
That she repeals him for her body’s lust;
And by how much she strives to do him good,
She shall undo her credit with the Moor.
– William Shakespeare
Othello, Act 2, Scene 3. Iago’s devious plan to bring down Cassio, Desdemona and Othello is revealed in one of the most important passages in the play. He uses a disease metaphor to describe how he will whisper poisonous words into Othello’s ear as Desdemona pleads Cassio’s case to her husband. The treacherous Iago aims to use Cassio’s honesty and Desdemona’s good nature to cast her as an adulteress with Othello’s sacked lieutenant. In this soliloquy Iago is delighted with his own badness.