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They stood by and watched their children die.

One after another, Dr. Paul Offit described the parents who prayed instead of getting their children the medical are they needed.

Herbert and Catherine Schaible prayed and prayed, but their 2-year-old son Kent died of pneumonia in Philadelphia 2009. It was bacterial pneumonia, and antibiotics could have saved him. They were convicted of child endangerment and involuntary manslaughter and placed on probation but horribly, the same thing happened again just four years later. In 2013, their 8-month old son Brandon died, again of bacterial pneumonia.

“We believe in divine healing, that Jesus shed blood for our healing and that he died on the cross to break the devil's power,” Herbert Schaible said in a 2013 police statement. Medicine, he said, “is against our religious beliefs.”

“When you see death, that’s the tip of the iceberg."

This time the Schaibles were charged with third-degree murder, pleaded no contest and were jailed. Their remaining children went into foster care. But all too often, Offit writes, such cases aren’t even investigated.

“When you see death, that’s the tip of the iceberg,” Offit told NBC News. “It’s the tip of the iceberg of a much larger degree of suffering.”

Offit chronicles these and many other cases in a new book, “Bad Faith”, which comes out Tuesday. He tells of Wesley Parker, who died at age 11 from type 1 diabetes after his parents withheld insulin and sang hymns instead. He writes about Terrance Cottrell Jr., who suffocated in 2003 during an exorcism.

“You want to understand how could a parent buck their most primal instinct,” Offit told NBC News.

“As a parent, you will do anything to save your child. That is your instinct. And yet here are people who are willing to stand by and watch their child suffer. I just wanted to understand it.”

He calls out a small group of Orthodox Jews in New York who continue to practice a form of circumcision that involves mouth-to-penis contact that’s led to the deaths of at least two infant boys from herpes, a virus that lives in and on the mouth.

Offit, a pediatrician and chief of infectious diseases at Children's Hospital of Philadelphia, thinks it’s time for legislators to crack down on parents who neglect their children’s health in the name of religion.

The ongoing outbreak of measles linked to Disneyland has started people talking about laws requiring vaccination. The Centers for Disease Control and Prevention says 140 cases are linked to the outbreak and says it’s taken hold because of pockets of parents who choose not to vaccinate their kids, or who choose to delay their vaccinations.

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Most of these vaccine doubters don’t say religion factors into their decision. But most states allow parents to opt their kids out of vaccines because of religious objections. Offit believes all such objections should be banned. Only kids with medical exemptions should go unvaccinated, he believes.

“How do we allow this to happen?” asks Offit, perhaps America’s most prominent advocate of vaccination. Mainstream religions, Offit says, embrace medicine. Yet American lawmakers continue to protect what he calls “delusional cults”.

“It doesn’t happen in England. It doesn’t happen in Canada. You will go to jail and you will lose your children,” he said. He points to Oregon’s 2011 law that eliminates religious exemptions to criminal charges after 83 children of people belonging to the Followers of Christ Church had died, some of them from documented treatable conditions such as diabetes and one from an agonizing hernia.

“And there hasn’t been a single death of a child among the Followers of Christ Church in Oregon since,” Offit writes.

“When it happened in Oregon, people changed their behavior,” he argues.

“You need to draw a line between what is a deeply felt religious belief and what is a dangerous, delusional cult.”

Some states are acting as havens for people who say they believe that God will heal the sick and that medical intervention is not only unnecessary, but contrary to God’s will, Offit says.

Some Followers of Christ Church moved to Idaho, he says, “which has a religious exemption to manslaughter and where the death rate among the children of the Followers is 10 times greater than in the general population.”

“You need to draw a line between what is a deeply felt religious belief and what is a dangerous, delusional cult.”

Pennsylvania may have convicted the Schaibles, but Offit says legislation did not follow.

“Remarkably, while Brandon Schaible lay dying, the General Assembly of Pennsylvania continued to protect parents who use a religious defense to medical neglect charges,” Offit writes.

“When it comes to religious exemptions to child abuse and neglect, Pennsylvania laws are particularly troubling – it is one of only two states that allow a religious exemption to bicycle helmets, and one of only a handful of states that allow parents who refuse medical care to adopt children.”

Paul Offit, defender of vaccine safety, in his office at Abramson Research Center at CHOP. Basic Books

Offit doesn’t believe parents who pray instead of taking their kids to doctors are deliberately hurting them.

He blames religious leaders who convince parents that prayer is more powerful than medicine, and groups that deliberately raise children to be ignorant of medical facts. Rita Swan, a Christian Scientist whose son Matthew died of treatable meningitis in 1977 as fellow church members threatened to reject her if she took him to the hospital, says she was never taught about antibiotics or vaccines.

Swan now advocates for medical treatment over prayer. “You never get past the guilt,” he quotes her as saying. “Religion has to serve the good of humanity.”

The U.S. spirit of “don’t tell me what to do” is a factor, also, Offit says.

“America was founded as a safe haven for religious beliefs that weren’t tolerated elsewhere,” he points out. “Unfortunately, the America public’s instinctive tolerance for religion often exceeds reason.”

Some people can be persuaded with facts, Offit says. “I think you can’t just put people aside and say they are stupid,” he said.

At the same time, laws need to be clear. “There are examples of legislation against religious practice,” he points out. Anti-polygamy laws are one example, as are laws that prevent Native Americans from smoking peyote.

And there are some people who Offit still doesn’t understand, even after delving into the history of people who believe in the power of prayer over medicine.

“If a car was coming down the street and someone’s child was standing there, I would imagine they would pull their child out of harm’s way,” he said.

“How is this any different?”