How is Mercutio's quote, "A plague o' both your houses," in Act 3, Scene 1 of Shakespeare's Romeo and Juliet, important to the story?

Mercutio's repeated line, "A plague o' both your houses!,"
is important because it curses both the Capulet and Montague families but it also points
out just how cursed they already are (III.i.90, 99-101). The fact that the repeated line
is spoken by Mercutio is absolutely critical to the story as well in that Mercutio is
connected to both families either as a relation or a freind.  Hence, only Mercutio
serves as the social bridge between the two families and now both families have killed
that one social bridge.

The word "plague" can refer to any highly
infectious disease that causes many deaths through an epidemic. It can also refer to any
"evil" that one can use to hurt another person with, therefore, we can think of a plague
as a curse (Random House Dictionary). Hence, one thing that Mercutio means when he says,
"A plague o' both your houses," is that he is cursing their families. We see this curse
come true when both Romeo and Juliet later die in the story. As Prince Escalus later
points out, God has punished Lords Capulet and Montague for their hatred by taking away
what they both loved (V.iii.303-304).

Since a plague is an infectious
disease that causes many deaths, we can also say that through this line Mercutio is
pointing out what he already sees as true, that both houses are already cursed due to
their hatred and prolonged feud. He is pointing out that their hatred is an evil that
has already shed a great deal of blood and is likely to shed even more, which it does,
as we see.

Finally, this line is especially important because it is
spoken by Mercutio who serves as a social bridge between both Capulets and Montagues and
even other citizens of Verona. Mercutio is Romeo's best friend; however, we also learn
that he is either related to or a close friend of the Capulets because Lord Capulet has
invited him to his feast, as we see from the line in the invitation list Romeo reads
that is handed to him by Capulet's servant, "Mercutio and his brother Valentine"
(I.ii.70). Not only that, Mercutio is also a family member of Prince Escalus's as we see
Romeo proclaim when Mercutio is slain:

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This gentleman, the Prince's near
ally,
My very friend, hath got this mortal hurt
In my behalf.
(III.i.109-111)

In this
passage the term "ally" can be translated as a relation, especially through marriage
(Random House Dictionary). Hence we know that Mercutio is also a relation of Prince
Escalus's through marriage. Not only that, we also learn later that Mercutio is a family
member of County Paris's, which we learn after Romeo slays Paris at Juliet's tomb and
cries out, "Let me peruse this face. / Mercutio's kinsman, noble County Paris!"
(V.iii.74-75). Hence, we see that Shakespeare has very intentionally made Mercutio a
relation, either as a friend or a relative, of every major character in the play,
creating a social bridge between the characters, especially the Capulets and Montagues.
Mercutio's death and his pronouncement of the plague show us just how much death and
destruction the feud is causing.