Juana trusts the doctor when she shouldn't.

Juana’s greatest weakness is that she is too trusting.  She tries to get the doctor for her baby.  When Coyotito is attacked by the scorpion.  Kino panics, grinding the scorpion into dust instead of helping the baby.  Juana, on the other hand, takes action.  She sucks the poison out of the wound.

It never occurs to most people in their village to call for a doctor, but Juana does. 

The word was passed out among the neighbors where they stood close-packed in the little yard behind the brush fence. And they repeated among themselves, "Juana wants the doctor." A wonderful thing, a memorable thing, to want the doctor. To get him would be a remarkable thing. (Ch. 1) 

This is a mistake because the doctor does not have the best interests of the people at heart.  He considers them animals, not people.  When he learns that Kino has no money, he pretends to not be home.  Then, when he learns that Kino has found a gigantic pearl, he suddenly changes his mind.

Kino is suspicious, because the baby seems fine.  The doctor says that he will get worse later. Kino nods to Juana, and she gives the doctor the baby.  He gives Coyotito a white powder.   The baby does indeed get worse.  The doctor comes back, and Juana assumes that the doctor will help her baby.

The doctor put the baby in Juana's arms. "He will get well now," he said. "I have won the fight." And Juana looked at him with adoration. (Ch. 3)

Juana thinks that the doctor is going to cure her baby. She does not realize that he is the one who made the baby sick in the first place.  All the doctor cares about is the giant pearl. When they had no money, he didn’t care about the baby at all.  Now all of a sudden he is telling everyone he is the baby’s doctor.