After the doctor initially dismisses Kino's concerns about his son's well-being, he hears the news of Kino's discovery and sees Coyotito's illness as an opportunity to make money. When the doctor initially visits Kino's hut, Coyotito's wound is almost completely healed. However, the doctor mentions that a scorpion's sting has a curious effect on the victim and the symptoms can return even stronger than before. Unfortunately, Kino is at the mercy of the doctor and realizes that he cannot take a chance of putting his ignorance against the doctor's possible knowledge. The doctor then proceeds to give Coyotito a capsule filled with white powder. The doctor then warns Kino and Juana that the poison will attack in an hour and promises to return to save the child's life. It is implied that the doctor purposely poisons Coyotito by giving him the mysterious capsule, and the baby becomes extremely sick within the hour. The doctor then returns to Kino's hut and "cures" Coyotito by giving him a glass of water with three drops of ammonia in it.

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When the doctor first comes to Kino and Juana's hut, he is aware of the fabulous pearl Kino has found and is already imagining what he could do with such a valuable payment for his services. "The news came to the doctor…And the doctor's eyes rolled up a little in their fat hammocks and he thought of Paris."

It was important, therefore, for the doctor to find a reason to treat Coyotito, even though the swelling in the baby's shoulder had subsided - Juana's treatment with the seaweed had been effective. However, the doctor examines the wound carefully and makes a show of giving Coyotito a capsule containing a white powder. Kino is suspicious of the doctor's actions but recognizes his ignorance and inability to argue with the treatment. The doctor informs Kino and Juana that the poison from the scorpion sting may return within the hour as he leaves their home.