Seven years after Americans celebrated the licensing of Jonas Salk’s polio vaccine, President John F. Kennedy called on Congress to finance a nationwide vaccination program to stamp out what he called the “ancient enemies of our children”: infectious disease.

Now Kennedy’s nephew, Robert F. Kennedy Jr., is the nation’s chief critic of vaccines — a public health intervention that has saved millions of lives — and President-elect Donald J. Trump’s pick to become the next secretary of health and human services. Mr. Kennedy calls himself a vaccine safety activist. The press calls him a vaccine skeptic. His detractors call him an anti-vaxxer and a conspiracy theorist.

Whatever one calls him, Mr. Kennedy is a polarizing choice whose views on certain public health matters beyond vaccination are far outside the mainstream. He opposes fluoride in water. He favors raw milk, which the Food and Drug Administration deems risky. And he has promoted unproven therapies like hydroxychloroquine for Covid-19. His own relatives called his presidential bid “dangerous for our country.”

If there is a through line to Mr. Kennedy’s thinking, it appears to be a deep mistrust of corporate influence on health and medicine. In some cases, that has led him to support positions that are also embraced by public health professionals, including his push to get ultra-processed foods, which have been linked to obesity, off grocery store shelves. His disdain for profit-seeking pharmaceutical manufacturers and food companies drew applause on the campaign trail.

People close to him say his commitment to “make America healthy again” is heartfelt.

“This is his life’s mission,” said Brian Festa, a founder of We the Patriots U.S.A., a “medical freedom” group that has pushed back on vaccine mandates, who said he has known Mr. Kennedy for years.

But like Mr. Trump, Mr. Kennedy also has a tendency to float wild theories based on scanty evidence. And he has hinted at taking actions, like prosecuting leading medical journals, that have unnerved the medical community. On Friday, many leading public health experts reacted to his nomination with alarm.

“This is the first time we’ve ever had someone that walking in the door whose public views, you just can’t trust,” said Dr. Georges Benjamin, the executive director of the American Public Health Association, which represents 25,000 public health professionals.

He called Mr. Kennedy, an environmental lawyer, “totally unprepared, by skill and by training,” for the job of secretary, which involves managing a department with more than 80,000 employees and 13 operating divisions, overseeing everything from Medicare to biomedical research.

Mike Pence, the former vice president under Mr. Trump, publicly opposed Mr. Kennedy’s nomination on Friday, citing his support for abortion rights.

The pharmaceutical industry’s trade group issued a statement neither praising nor criticizing Mr. Kennedy’s nomination. Calling itself “a crown jewel of the American economy,” the group vowed to “work with the Trump administration to further strengthen our innovation ecosystem and improve health care for patients.” Shares of stock in vaccine makers fell after Mr. Trump announced his selection of Mr. Kennedy on Thursday.

Mr. Kennedy did not respond to a request for an interview, and he has not publicly outlined his priorities. But if he is confirmed by the Senate, he would have wide latitude as health secretary, and has forecast his plans to shake things up.

In an interview in January 2024 with Dr. Mark Hyman, before he suspended his own independent presidential campaign and endorsed Mr. Trump, he spoke about what he would do if elected, outlining some of his lesser-known goals.

He said he would steer the nearly $48 billion annual budget of the National Institutes of Health away from drug development and toward studies that would explain high rates of chronic disease. He also said he would seek to prosecute medical journals like The Lancet and The New England Journal of Medicine under the federal anti-corruption statute.

“I’m going to litigate against you under the racketeering laws, under the general tort laws,” he said during the interview. “I’m going to find a way to sue you unless you come up with a plan right now to show how you’re going to start publishing real science and stop retracting the real science and publishing the fake pharmaceutical science by these phony industry mercenaries, scientists.”

More recently, Mr. Kennedy has said Mr. Trump would advise communities to stop fluoridating their water, contradicting the advice of Centers for Disease Control and Prevention, which cites fluoridation to reduce tooth decay as on of the ten great public health achievements of the 20th century. He has said he would slash 600 jobs at the N.I.H. and has instructed Food and Drug Administration officials to “preserve your records” and “pack your bags.”

Mr. Kennedy has argued that the F.D.A. is a victim of “corporate capture.” Under a three-decade-old “user fee” program, drug, device and biotech companies make payments to the agency partly to seek product approvals. The fees now account for nearly half the F.D.A.’s budget.

“The F.D.A. is just a sock puppet to the industries it is supposed to regulate,” Mr. Kennedy said in the interview with Dr. Hyman. “All of this is easily changed. I’m not saying I’m going to be able to accomplish it all on Day 1, but I’m going to accomplish it very quickly.”

Public health experts typically focus on Mr. Kennedy’s views on vaccination. Mr. Kennedy has cast doubt on Covid vaccines and has promoted the long-debunked theory that vaccines cause autism. His detractors say that his critiques of vaccine safety have cost lives, pointing in particular to a visit Mr. Kennedy made to Samoa in 2019.

That year, the island nation put its measles vaccination program on hold after the death of two infants who had received measles shots. Mr. Kennedy visited and, according to news reports, met with a prominent vaccine opponent, giving a boost to the anti-vaccination movement there.

The deaths were subsequently attributed to a mistake by the nurses who administered the vaccine, not to the vaccine itself. But the dip in vaccination rates led to a measles outbreak and 83 deaths.

Dr. Jonathan E. Howard, an associate professor of neurology and psychiatry at New York University who has been tracking the anti-vaccine movement for the past 15 years, said Mr. Kennedy would be “a disaster” as health secretary, adding, “He is an anti-vaccine paranoid crank who has a trail of dead children in Samoa.”

Often, Mr. Kennedy uses a “we don’t know” construction to spin out unproven theories, such as his suggestion that the coronavirus might have been engineered to attack specific races.

“Covid-19 is targeted to attack Caucasians and Black people,” Mr. Kennedy said at a fund-raiser, according to The New York Post. “The people who are most immune are Ashkenazi Jews and Chinese.”

“We don’t know whether it was deliberately targeted or not, but there are papers out there that show the racial or ethnic differential and impact,” he added.

Mr. Kennedy was apparently referring to a 2020 study looking at genetic differences in Covid-19 patients, but numerous studies examining racial disparities attribute them to socioeconomic factors, including poverty and lack of access to health care.

Mr. Kennedy also has repeatedly suggested that chemicals in the water might be responsible for “sexual dysphoria” in children. In a clip aired by CNN, he noted that atrazine, a herbicide sometimes found in well water in agricultural areas, will “concentrate and forcibly feminize” frogs. He went on, “What this does to sexual development in children, nobody knows.”

There is no evidence that the chemical, typically used on farms to kill weeds, causes the same effects in children, although studies show it has been linked to birth defects in babies whose mothers are exposed to it. But according to the Agency for Toxic Substances and Disease Registry, an arm of the C.D.C., “Most people are not exposed to atrazine on a regular basis.”

Mr. Kennedy came to health advocacy through his work as an environmental lawyer. In 1999, he was named a hero of the planet by Time magazine for his work with the Riverkeeper organization, among the groups credited with cleaning up New York’s polluted Hudson River. One of his aims was to get mercury out of waterways.

In a speech last year at Hillsdale College, he said that in 2005, when he was giving speeches around the country, mothers of children with autism approached him to ask him to take a look at vaccines, some of which had in the past contained a mercury-based preservative, thimerosal. The preservative was removed by manufacturers in 2001 at the request of the Centers for Disease Control and Prevention, and subsequent studies showed no link between the preservative and autism.

In the years since, Mr. Kennedy has made millions of dollars railing against vaccines. The windfall has come through books, including one denigrating Dr. Anthony S. Fauci, the former government scientist both celebrated and despised for his work on Covid, and through Children’s Health Defense, the nonprofit he has used as a platform to sow doubts about vaccination.

Allies of Mr. Kennedy say he has earned the right to make policy, given the support he generated while campaigning alongside Mr. Trump. Calley Means, a health care entrepreneur who has been an adviser to Mr. Kennedy and who was instrumental in connecting him to Mr. Trump, said in an interview last week that Mr. Kennedy had “a true mandate to take on broken health care institutions, and to deliver the change.”