John of God cautions that cures are not always instantaneous, but can take months or years and the entities cannot heal everyone. Some may be just too sick; others may not be ready spiritually.
When patients come before him, he makes a diagnosis with just a glance -- scribbles a prescription for herbs or even schedules an operation.
Some surgeries are "invisible." The entities are said to have such supernatural powers, they can heal without breaking the skin. Others are "visible" -- and only certain patients are considered eligible. They must volunteer, be 18 to 52 years old, and cannot be in wheelchairs, or have recently had radiation or chemotherapy.
The "visible" surgeries can be graphic. "Primetime" witnessed one in which João took four-inch gauze-tipped steel forceps, dipped them in a solution he calls "holy water," and shoved the forceps all the way up a patient's nostril and twisted them violently.
It took 45 seconds, and the patient left bleeding. But João's assistants videotape such surgeries regularly and sell them at the gift shop.
It's against the law to practice medicine without a license in Brazil. "John of God" has been charged, fined and even jailed briefly. He keeps on performing surgeries, saying it's the entities, not him, at work.
About the surgeries, he said: "I don't do that. God and the spirits do that." He says even looking at the videotapes of the surgeries makes him queasy.
He says he doesn't even remember the experience. "I am unconscious," he told "Primetime Live's" John Quiñones. He likened his state to being asleep.
Challenged over the propriety of these operations, João answered, "Bring your scientists here, bring the doctors, bring them here. There is no magic going on. Just the power of God."
Some people say the healings are just a front -- a way to make John of God rich.
Even though he charges no fee for treatment, João appears to be a wealthy man. He owns a cattle ranch just down the road from where he sees patients -- more than 1,000 acres.
When Quiñones pointed out to João that his town has become a tourist beacon with thousands coming to spend money for herbs and other items, he looked hurt. His eyes turned red and watered.
He said he has money but he spends it to pay for food and education for the poor. "I have cattle, but that's not enough to keep the casa," he said.
Yet, there are also rumors that John of God has a much darker side. Juliana Almeida Franca, a district attorney who has investigated John of God, says he sent her death threats -- delivered by a relative. João denies this.
João has also been accused of taking advantage of a woman who came to be healed. "There is a lot of jealousy. People talk. What dictates is the conscience toward God," he answered.
He insisted his healings are legitimate. "You can fool the people for one to two years. But you cannot fool people for 45 years," he said.
For a second opinion, "Primetime" consulted Dr. Mehmet Oz, one of the most respected surgeons in the United States.
Asked about Ireland's remarkable improvement from a brain tumor, he agreed, "something stopped a process that normally is very aggressive." He also wondered if the atmosphere of John of God's clinic contributed.