Who yells a plague both your houses in the book romeo and juliet?

The line "A plague o' both your houses!" is yelled by Mercutio in Act III, Scene 1 of Romeo and Juliet.

He utters this curse after being fatally wounded by Tybalt, the cousin of Juliet's husband, Romeo. Mercutio is deeply hurt by the feud between the Montagues and Capulets, and blames both families for his death.