As the parents watch the ominous scorpion, Kino hears the "Song of Evil" and Juana repeats "an ancient magic to guard against such evil" and finishes with a Hail Mary between "clenched teeth."
The fear of Juana is an ancient fear: the scorpion contains poison that can kill; moreover, the Indians are not often afforded medical care because of their inability to pay for this attention. Therefore, when the scorpion stings her baby, Juana immediately reacts by trying to suck out the poison and spitting it out because she knows that to get the doctor
...would be a remarkable thing. The doctor never came to the cluster of brush houses.
This pivotal moment generates the other conflicts that move the plot of Steinbeck's parable.