As Mercutio is dying he curses both households by wishing "A plague o' both" their "houses." Three times. Repeating the curse three times Shakespeare wants to show the viewer and make them realize the impact of Romeo and Tybalts actions (In the 15 century to do something three times was considered to invoke magical powers which would make whatever wish action to become true, this is where the expression"three is the magic number" comes from -page 237 "will in the world") and shows why Mercutio does not forgive Romeo just before he dies, and this brings the viewer to fully understand the how dark a mood Mercutio is bring the play.
Benvolio and Mercutio (another one of Romeo's friends) are waiting on the street later that day when Tybalt arrives. Tybalt demands to know where Romeo is so that he can challenge him to a duel, in order to punish him for sneaking into the party. Mercutio is eloquently vague, but Romeo happens to arrive in the middle of the verbal sparring. Tybalt challenges him, but Romeo passively resists fighting, at which point Mercutio jumps in and draws his sword on Tybalt. Romeo tries to block the two men, but Tybalt cuts Mercutio and runs away, only to return after he hears that Mercutio has died. Angry over his friend's death, Romeo fights with Tybalt and kills him. Then, he decides to flee. When Prince Escalus arrives at the murder scene, he banishes Romeo from Verona forever.