Most American parents hardly give thought to polio beyond the instant their child is immunized against the disease. But there was a time in this country when polio paralyzed 20,000 people in a year, killing many of them.

Vaccines turned the tide against the virus. Over the past decade, there has been only one case in the United States, related to international travel.

That could change very quickly if polio vaccination rates dropped or the vaccine were to become less accessible.

Robert F. Kennedy Jr., a longtime vaccine skeptic who may become the secretary of health and human services, has said the idea that vaccination has nearly eradicated polio is “a mythology.”

And while Mr. Kennedy has said he’s not planning to take vaccines away from Americans, he has long contended that they are not as safe and effective as claimed.

As recently as 2023, he said batches of an early version of the polio vaccine, contaminated with a virus, caused cancers “that killed many, many, many, many, many more people than polio ever did.” The contamination was real, but research never bore out a link to cancer.

Aaron Siri, a lawyer and adviser to Mr. Kennedy, has represented a client seeking to challenge the approval or distribution of some polio vaccines on the grounds that they might be unsafe.

Those efforts appear unlikely to succeed. And there is widespread support for vaccination among prominent Republicans, including President-elect Donald J. Trump and Senator Mitch McConnell, who had polio as a child.

But the secretary of health and human services has the authority to discourage vaccination in less direct ways. He or she could withdraw federal funds for childhood vaccination programs, hasten the end of school mandates in states already disinclined toward vaccines or fuel doubts about the shots, exacerbating a decline in immunization rates.

If polio vaccination rates were to fall, scientists say, the virus could slip into pockets of the country where significant numbers of people are unvaccinated, wreaking havoc once more. The virus may be nearly eradicated in its original form, but resurgence remains a constant threat.

Any decision the Trump administration makes regarding the polio vaccine is likely to ripple across the globe, said Dr. David Heymann, an infectious disease physician at the London School of Hygiene and Tropical Medicine and former leader of polio eradication at the World Health Organization.

“If the U.S. takes away the license, then many other countries will do the same thing,” he said. To have polio resurge when it is so close to eradication “would be very, very, very, very sad.”

Before 1955, when the vaccine was introduced, polio disabled more than 15,000 Americans each year and hundreds of thousands more worldwide. In 1952 alone, it killed 3,000 Americans after paralysis left them unable to breathe.

Many of those who survived still live with the consequences.

“People really underestimate how horrific polio was,” said Dr. Karen Kowalske, a physician and polio specialist at University of Texas Southwestern Medical Center in Dallas.

Many who recovered now suffer “post-polio syndrome”: Some of the original symptoms, including muscle weakness and respiratory problems, return.

Dr. Kowalske tends to about 100 post-polio patients who need braces, wheelchairs or other devices to cope with progressive weakness. Some are older adults who became infected before the vaccine was available; others are middle-aged immigrants from countries where polio remained a problem for much longer than in the United States.

To some survivors, the idea of polio’s return is unfathomable.

Carol Paulk contracted the disease in 1943, when she was just 3. Her right leg never recovered, and for the rest of her life she has walked with a pronounced limp and has been in near-constant pain.

Ms. Paulk is among the luckier ones. Until recently, she did not suffer the breathing, swallowing or digestive problems that often torment polio survivors.

She has had “a wonderful, wonderful life” with a husband and three daughters, a law degree and extensive travel abroad.

But always, everywhere, she is calculating how far away the next seat is, how long her energy will hold out and whether a given activity is worth debilitating pain the next day.

She didn’t participate in the 1963 March on Washington or play sports, as she desperately wanted to, or go hiking, skiing and bicycling with her husband.

If there were a public hearing about the polio vaccine now, “I would go, and I would take off my brace, and I would let them see my leg and ask them, is that what they want for their children?” she said.

Polio disables many fewer children now. Vaccination has scrubbed the virus from most of the planet, slashing the number of cases by more than 99.9 percent and preventing an estimated 20 million cases of paralysis.

Still, the virus has turned out to be a stubborn enemy, and eradication has been set back over and over again.

In 2024, 20 countries reported polio cases, and the virus was detected in wastewater in five European countries, decades after its official elimination from the region, and in Australia.

“Any reduction in coverage rates increases the risk of polio anywhere,” said Oliver Rosenbauer, a spokesman for the World Health Organization’s polio eradication program.

There are three types of polioviruses, and eradication requires that all three disappear. For years, the goal has been tantalizingly close.

Type 2 was declared vanquished in 2015, and Type 3 in 2019. Type 1 now circulates only in Afghanistan and Pakistan. In 2021, the two countries together had just five cases; in 2024, they had 93.

But those figures tell only part of the story. In a surprising twist, an oral vaccine used in some parts of the world has kept poliovirus circulating long after it should have died out.

In most low- and middle-income countries, health officials still rely on an oral vaccine given as two drops on the tongue. It is inexpensive and easy to administer, and it prevents transmission of the virus.

But it contains weakened virus, which vaccinated children can shed into the environment through their feces. When there are enough unvaccinated children to infect, the pathogen slowly spreads, regaining its virulence and eventually causing paralysis.

The problem is this: Since 2016, the oral vaccine used for routine immunization has not protected against Type 2 virus. Global health authorities made a deliberate decision to reformulate the vaccine on the grounds that naturally occurring Type 2 virus had disappeared.

That turned out to be premature. More Type 2 virus had been shed by orally vaccinated children in some parts of the world than officials had anticipated. When some nonimmunized children, or those given the newer oral vaccine, encountered this “vaccine-derived” Type 2 virus, they became infected and paralyzed.

Vaccine-derived poliovirus now paralyzes more children than naturally occurring virus does. For example, Nigeria eliminated all so-called wild-type polio in 2020. But in 2024, the country saw 93 cases of Type 2 vaccine-derived virus, more than one-third the global total.

None of this is a problem for Americans — as long as they are vaccinated.

The inactivated polio vaccine (I.P.V.) used for routine immunization of American children protects against all three types of polio. These formulations contain dead virus, and so cannot cause disease or revert to a dangerous form.

But like some other vaccines for infectious diseases, they do not fully prevent infection or transmission of the virus. This aspect is among the criticisms of Mr. Siri, Mr. Kennedy’s adviser.

Still, it is less important than the vaccines’ near-perfect power to prevent paralysis, experts said.

“Yeah, yeah, it’s true, I.P.V. doesn’t prevent transmission,” said Dr. William Petri, an infectious diseases physician and past president of the W.H.O.’s polio research committee. “But, boy, that’s the best thing since sliced bread at preventing paralysis.”

It does mean, however, that people vaccinated with I.P.V. can keep the virus circulating, even when they themselves are protected against illness and paralysis.

So here’s a realistic scenario that worries researchers: Someone who was vaccinated with the oral polio vaccine in another country might bring the virus into the United States and then shed it, in its weakened form. This has already happened in other countries.

So long as most of the population remains vaccinated, this is not likely to set off an epidemic. But if the virus makes its way into communities with low vaccination rates, it may spread, and then revert to a virulent form that can cause paralysis.

That is what happened in New York in 2022, when polio struck a 20-year-old unvaccinated member of an ultra-Orthodox Jewish community in Rockland County.

The vaccination rate in that county was just over 60 percent, compared with the national average of 93 percent.

The virus that paralyzed the young man had been circulating for months, and it was later detected in the sewage of multiple New York counties with vaccination rates hovering around 60 percent, prompting the state to declare an emergency.

Genetically related polioviruses were detected in wastewater samples in Britain, Israel and Canada, suggesting widespread transmission. The authorities later found two distinct vaccine-derived Type 2 polioviruses in New York wastewater, suggesting two separate importations.

If polio were to re-emerge in the United States, it is unlikely to be as horrific as it was in the pre-vaccine decades. Many older adults still remember that as children they were not permitted to swim in rivers or pools, or anywhere the virus might lurk.

“The reason we weren’t allowed to play in rivers in the ’50s is because raw sewage was dumped into the rivers,” Dr. Heymann said.

That is no longer the case, so there “wouldn’t be massive transmission immediately in the U.S.,” he added.

But even if just a few children were to become paralyzed, “it would be awful.”

“We will make Americans healthy again,” Robert F. Kennedy Jr. has declared. A political action committee that has promoted Mr. Kennedy, President-elect Donald J. Trump’s pick for health and human services secretary, says his movement is “igniting a health revolution in America.”

But the word “again” presumes a time in the country’s past when Americans were in better health. Was there ever really a time when America was healthier?

For historians of medicine, there is a short answer.

“No,” said Nancy Tomes, a historian at Stony Brook University.

John Harley Warner, a historian at Yale, said, “It’s hard for me to think of a time when America, with all the real health disparities that characterize our system, was healthier.”

Dr. Jeremy Greene, a historian at Johns Hopkins University, asked: “Which particular era does R.F.K. want to take us back to?”

Probably not the 19th and early 20th century.

Rich men smoked cigarettes and cigars, the poor chewed tobacco. Heavy drinking was the norm.

“It was definitely a drinking culture,” said Dora Costa, an economic historian at the University of California, Los Angeles. “Drinking was a huge problem, saloons were a huge concern. Men were drinking away their paychecks. That is the reason we had Prohibition.”

And, Dr. Costa notes, American diets for most of the 19th century were monotonous.

It’s true that agriculture at the time was organic, food was locally produced and there were no ultraprocessed foods. But fresh fruits and vegetables were in short supply because they were difficult to ship and because growing seasons were so short. For the most part, Dr. Costa said, until the 1930s, “Americans were living off of dried fruits and vegetables.”

As for protein, Americans were relying on salted pork, she said, because meat was difficult to preserve. Only after the Civil War did meatpackers in Chicago begin to process meat and ship fresh beef across the country. At that point, Dr. Costa said, beef “became a large part of the American diet.”

But even though the availability of beef helped diversify diets, people did not become healthier.

Dr. Costa worked with Robert Fogel, the University of Chicago economic historian and Nobel laureate, to understand the health of a population of Americans living in the North around this period by examining the medical records of Union Army soldiers. Common conditions, like hernias, were untreatable — men had hernias as big as grapefruits, held in by trusses. Nineteen percent of those soldiers had heart valve problems by the time they were 60, compared with about 8.5 percent today.

Poor nutrition led to poor health. People were thin, often too thin. In 1900, 6.1 percent of Union Army veterans were underweight — a risk factor for various illnesses and often a marker of ill health — compared with 1.6 percent of U.S. adults today. In 1850, males at age 20 could expect to live to around 61 years. Today it is 74 years.

The start of the 20th century saw public health improvements (cleaner water, for example, and posters advising parents not to give their babies beer), but disease was rampant. There were no antibiotics and very few vaccines. When the 1918 flu struck the nation, no one knew the cause — the flu virus had not been discovered and strange folk remedies were rampant. About 675,000 Americans died. In 1929, the Great Depression began, and its economic toll over the next decade led to severe nutritional and health problems.

Health improved in the second half of the 20th century but was poor compared with that today.

Many people are nostalgic for the 1950s and 1960s, seeing those decades as a time of prosperity, when the American pharmaceutical industry pumped out new medical advances: antibiotics, antipsychotics, drugs for high blood pressure and vaccines for tetanus, diphtheria, measles and polio.

Despite that progress, those years were terrible for health, Dr. Greene said, with “a tremendous amount of heart attacks and strokes.”

Heart disease was rampant in 1950, with 322 deaths per 100,000 Americans annually from cardiovascular disease, double the rate today. By 1960, Dr. Greene said, heart disease, was responsible for one-third of all deaths in America.

In part, that was because nearly everyone smoked.

“We were among the heaviest smoking countries,” said Samuel Preston, an emeritus professor of sociology at the University of Pennsylvania.

Dr. David F. Musto, a medical historian at Yale, who died in 2010, once said in an interview that although he never enjoyed smoking, the social pressure to smoke when he was in college in the 1950s was so great that “I felt it was my duty to find my brand.”

Smoking greatly increases the risk for heart disease, the leading killer in the 1950s and 1960s.

Heart disease death rates plummeted in recent decades because smoking is much less common now, and treatment for heart disease is much more effective. Cholesterol-lowering statins, introduced in 1987, reduced the risk of heart disease. Other new medications as well as bypass surgery and stents also saved lives.

Cancer was the second leading killer in the 1950s, as it is today. But in 1950, there were 194 cancer deaths per 100,000 people. Now there are 142 cancer deaths per 100,000 people.

A decline in smoking is a leading reason, but there also has been a revolution in cancer treatment.

Until the 1990s, cancer was treated with brute force: surgery, radiation and chemotherapy. Now an array of targeted therapies are turning some cancers, once deadly, into treatable chronic diseases or even curing them.

Dr. Greene said he was not surprised by the idea of a halcyon past when people were healthier.

“There’s a long history in America of nostalgia for a past that was better than the present,” he said. “History is all about erasure — the things we don’t choose to remember.”

Today is not a sort of health utopia, of course.

Researchers are quick to acknowledge that Americans’ health is not as good as it can be. And they bemoan the huge disparities in health care in this country.

Yet the U.S. spends more on medical care than other countries — an average of $12,555 per capita, which is about twice what other wealthy countries spend.

But, historians say, the past was actually much worse.

And so, they say, the phrase “Make America Healthy Again” makes no sense.

“As a historian of health, I don’t know what ‘again’ Kennedy is imagining,” Dr. Tomes said. “The idea that once upon a time all Americans were healthy is a fantasy.”