How is Juana's life changed by the scorpion bite in The Pearl?

After the scorpion bites her baby, Juana's simple, but relatively peaceful life becomes disturbed and complicated by reversals of fortune.

Before her baby is bitten by the scorpion, Juana is content with her simple life as peasant. She sings softly in the morning as she prepares a meal:

Juana sang softly an ancient song that had only three notes and yet endless variety of interval....Sometimes it rose to an aching chord that caught the throat saying this is safety, this is warmth, this is the Whole.

However, the "Whole" is shattered with the events subsequent to the incident of the scorpion. Then, the "Song of Evil" enters her family's lives. As a result of their fear of the baby's dying from the scorpion bite, Kino seeks the doctor, but when he sees that Kino is a peasant without money, the doctor tells his servant to say that he is out. An angered Kino strikes the gate with his fist, bringing blood to his knuckles.

After this rejection, Kino decides that he must dive deeper into the ocean than he usually does in order to find a pearl with which to hire a doctor. Ironically, while she waits on Kino, Juana makes a poultice of seaweed that is "as good a remedy as any" for the baby. Finally, Kino comes onto shore with a magnificent pearl. 
Thinking he has found "The Pearl of the World," Kino plans to return to the doctor. However, word of this pearl soon reaches others and they come to ask Kino for favors. The avaricious doctor pretends that the scorpion's bite, which appears to be healing, can become inflamed again, so he must treat it. When Kino tells him he can pay with the pearl he has found, the doctor acts as though he has no knowledge of this great pearl.
But that night the "Song of Evil" enters the home of Kino and Juana as a robber, who is probably sent by the doctor, sneaks around and, hearing him, Kino strikes "first blood."

Because they are but peasants, Kino and Juana are exploited. When Kino takes his pearl to the pearl dealers, who are really agents of the same dealer, they conspire and pretend that this beautiful pearl is worthless because it is over-sized. Their refusal to give him what he feels it is worth causes Kino to vow to go to Mexico City and sell it there. But their neighbors worry because other men who have tried to do the same thing have met harm or death.

"That good wife Juana...and the beautiful baby Coyotito, and the others to come. What a pity it would be if the pearl should destroy them all."

Their worries are prophetic. Despite Juana's efforts to stop him, Kino refuses to be cheated on this Pearl of the World. Then, after Kino kills a thief, Juana tells her husband,

"Kino, this pearl is evil. Let us destroy it before it destroys us. Let us crush it....Let us--let us throw it back in the sea where it belongs."

Nevertheless, Kino insists, "I will fight this thing." Tragically, however, Juana is wiser, for on their journey to the city, Kino kills another man, but not before the thief, believing that he has heard a coyote howl, shoots Coyotito as he wails in the night. After this tragic occurrence, Juana convinces Kino to walk with her to shore and toss back the pearl.

And the pearl was ugly; it was gray like a malignant growth.

Their simple, but loving and peaceful life broken by murders and the tragic loss of their baby, Juana and Kino's existence is forever altered.