In Act 3, Scene 1 of "Romeo and Juliet", while Mercutio is dying he curses both families, the Montague and the Capulets. He says "A plague o' both your houses", also transcribed as "A pox on both your houses". Mercutio curses these families because he thinks he has been mortally wounded as a consequence of their feud. So with those remarks, "A plague o'both your houses", Mercutio is cursing both Romeo's and Tybalt's families, the Montague and the Capulets.
