This article is a collaboration between The New Yorker and ProPublica.

On the morning of April 19, 2016, Melanie Lilliston received an urgent call from the Little Dreamers day-care center, in Rockville, Maryland. Her six-month-old daughter, Millie, was being rushed to the hospital. Doctors there found that Millie had fractured ribs, facial bruises, and a severe brain injury. Melanie watched as her daughter was loaded onto a helicopter for emergency transport to Children’s National medical center, in Washington, D.C., where doctors discovered more injuries: a fractured leg and arm, and bleeding in her eyes. Millie died three days later.

The day-care operator, Kia Divband, told police that Millie had started choking while drinking a bottle of milk and lost consciousness. The Montgomery County medical examiner, however, determined that her injuries were caused by blunt force. Investigators discovered, on Divband’s phone and computer, Internet searches for “broken bones in children” and “why are bone fractures in children sometimes hard to detect.” A former employer of Divband’s told them that, the day before Millie was hospitalized, Divband had called to inquire about a job, and a baby could be heard wailing in the background. Divband told him the baby wouldn’t stop crying and that “he just couldn’t take it anymore,” the former boss recalled. Divband was arrested and charged with fatally abusing Millie.

At Divband’s trial, last year, a radiologist named David Ayoub testified for the defense. Ayoub, who is a partner in a private radiology practice in Springfield, Illinois, told jurors that he had reviewed X-rays and other medical records, and had concluded that Millie had rickets, a rare condition that causes fragile bones. The disorder, which is usually brought on by a prolonged and severe lack of Vitamin D, could explain Millie’s injuries, Ayoub said.

Seeking to cast doubt on Ayoub’s credibility, the prosecutor brought up a different issue. Was it true, she asked, that Ayoub believed that Gavi, the Vaccine Alliance, a charity funded by the Bill and Melinda Gates Foundation to increase vaccination rates in poor countries, was committing genocide? “That’s right,” Ayoub said.

The prosecutor asked if Ayoub believed that Gavi—along with the World Health Organization, the Gates Foundation, and UNICEF—was using vaccinations to force sterilization on people in Third World countries. “Yes, that’s my belief,” Ayoub said.

As evidence, he cited a report from 1972 by a commission headed by the philanthropist John D. Rockefeller III and a 1974 study overseen by Henry Kissinger, who was the Secretary of State at the time, warning about the dangers of population growth. It’s “no leap of faith” to believe that vaccination is being used to carry out this agenda, Ayoub said.

The prosecutor also questioned Ayoub about a speech he delivered in 2005, in which he said that his views on vaccination—including his belief that it has contributed to a rise in autism—put him in a “fringe group,” and even in the “fringe of that fringe.” Ayoub acknowledged making the statement. “Thinking that vaccines were associated with autism, you’re clearly a minority view if you’re a physician,” Ayoub testified. “If you think it’s done intentionally for nefarious purposes, you’re clearly another level of—you know—different.”

In an e-mail, Ayoub said that he did not mean to accuse the Alliance or the Gates Foundation of intentional genocide, though he realized that his 2005 lecture might give that impression. “I was concerned by confirmed sporadic reports that some vaccines distributed in third-world countries contained fertility-reducing substances,” he said. “Regardless of whether this was deliberate, careless, unintentional or a cost-cutting measure, I felt that there was a potential for abuse and that this should be investigated.”

In the past decade, Ayoub, who is fifty-nine, has become one of the country’s most active expert witnesses on behalf of accused child abusers. He estimates that he has testified in about eighty child-abuse cases in the United States, Sweden, and the United Kingdom. He has consulted or written reports in hundreds more.

Prior to his child-abuse work, Ayoub was a prominent supporter of a movement that blames the rise in autism—a neurological and developmental disorder that starts in early childhood—on vaccinations that contain mercury, aluminum, or other substances. These claims are mostly dismissed by scientists, but they have nonetheless spurred a burgeoning worldwide “anti-vaxxer” movement, which has fuelled a decline in vaccination rates. Both positions reflect a deep suspicion of government and mainstream medicine and a rising backlash against scientific consensus in an era when misinformation quickly spreads online.

Ayoub, in a series of interviews, said that his criticism of vaccines is no longer a significant part of his work, and has no bearing on his credibility as a witness in child-abuse cases. (The Divband trial ultimately ended in a mistrial, after jurors could not agree on a verdict. Prosecutors later retried the case and Divband was convicted on child-abuse charges and sentenced to fifty years in prison; Ayoub did not testify in the second trial.) Ayoub said that his testimony in each abuse case is based on a careful review of the medical evidence. He simply wants to see justice done and does not charge for his services as an expert witness, he said. “Parents are being accused and families torn apart based on fractures and/or other boney irregularities that are in fact attributable to bone fragility, not abuse,” he said in an e-mail. If rickets, Vitamin D deficiency, and other explanations are not addressed, he added, “parents cannot receive fair trials, and families will be destroyed based on a misunderstanding of the radiology and pathology.”

Ayoub doesn’t specialize in treating children, however. He is not a pediatrician or a pediatric radiologist. Much of his knowledge about rickets in infants comes from reading studies and textbooks, he has said on the stand, rather than from formal training. His frequent diagnosis of rickets is questioned by specialists in the field. Peter Strouse, the chief of pediatric radiology at the University of Michigan’s C. S. Mott Children’s Hospital, who has served as a prosecution witness in about eight child-abuse cases, and has consulted in cases where Ayoub was a defense expert, described Ayoub’s views as “a complete fabrication. It’s sad they can get away with that in court.”

David Ayoub is sworn in as an expert witness in the James Duncan case. Photograph by Zack Wittman / ProPublica

Growing up in Peoria, Illinois, Ayoub was a track star who set the state high-school record in the eight-hundred-and-eighty-yard run, in 1977. As an undergraduate, at the University of Illinois, he was the Big Ten conference champion in the thousand-yard run, and he later attended medical school there. Following an interventional-radiology fellowship at the University of Iowa, Ayoub returned to Illinois in 1991 and began practicing radiology in Springfield.

Ayoub told me that he became interested in vaccines about fifteen years ago, after researching treatment for a bothersome knee. He was reading about alternative therapies and ended up subscribing to a newsletter from Joseph Mercola, a proponent of alternative treatments with a large online following and a Web site that frequently features pieces criticizing vaccination. Mercola has also promoted other controversial views, including the idea that fluoridated water can give children A.D.H.D. (In 2016, Mercola agreed to pay as much as $5.3 million in customer refunds to settle a complaint by federal regulators that he made false claims about the health benefits and safety of tanning beds he sold. Mercola did not respond to requests for comment.)