Why Does Mercutio Say "A Plague O' Both Your Houses"?

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In Shakespeare's tragedy, "Romeo and Juliet," Mercutio says, "A plague o' both your houses" because the feud between the Capulet and Montague families has led to the events that resulted in his death. He speaks these words in Act 3, Scene 1, and his words turn out to be very prophetic as the play unfolds.