How does Bruce persuade the witch doctor to use medicine for the pinkeye epidemic?

Bruce persuades the witch doctor to use medicine for the pinkeye epidemic. He deliberately infects himself with pinkeye. Then he goes to the witch doctor and has her try her incantations. When they do not work, he asks her to try using the Terramycin he has. She smears some in his eyes and also sings prayers that God would heal Bruce. In three days his eyes clear up, but everyone else is miserable. Several days later, he approaches the witch doctor and asks if she wants to try the Terramycin on the Motilones. She agrees and is able to cure everyone.