Church bells pealed. Car horns blared. The enemy that had laid waste to thousands was on the run, and on that morning 50 years ago, Americans rejoiced.

In a single, electrifying moment, they'd learned that Jonas Salk and a team of researchers had developed a vaccine that could conquer poliomyelitis, one of the most dreaded diseases of the 20th century.

The formula, a triple strain of dead polio virus, was an approach many scientists had expected would fail. The Salk team had tested it in the largest clinical trial in history, an experiment in which nearly 2 million youngsters took part. Now the results were in.

"The vaccine works," the announcement declared. "It is safe, effective and potent."

The news, a watershed in medicine, made Salk a national hero. And it changed life forever in cities large and small. It brought an end to summers of shuttered movie theaters, padlocked swimming pools and abandoned beaches, an end to the relentless onslaught of headlines reporting the latest polio epidemic. No longer would families be haunted by the specter of paralysis -- or death.

"The hope that it offered was tremendous," said John Sever, an infectious disease pediatrician at Children's National Medical Center who was then a Chicago medical student caring for young victims of the virus. He still can hear the sound of their iron lungs, the massive mechanical tubes that kept the most critically ill alive by forcing air in and out of their bodies.

"A terrible grinding, clinking noise," he recalled, softened only by the faint sibilance of leaking oxygen.

Fifty years later, the achievement of April 12, 1955, again is being celebrated: at the University of Pittsburgh, where Salk and colleagues labored feverishly for nearly a decade; at the University of Michigan, where their success was announced before an overflow crowd; in Boston, where others had done the groundbreaking research into how the virus could be grown in quantity; in La Jolla, Calif., home to the Salk Institute for Biological Studies.

Washington plans to mark the anniversary at the Smithsonian Institution. A gathering tomorrow with Salk's youngest son and the granddaughter of the disease's most famous casualty, Franklin D. Roosevelt, will herald Tuesday's opening of "Whatever Happened to Polio?" at the National Museum of American History. Like the other events, the exhibit will look backward and forward in exploring the advances the campaign brought about and the ethical issues it raised.

A Persistent Threat

The United States has been polio-free for more than a quarter-century, trailed by the rest of the Western Hemisphere, then by the countries of the Western Pacific and then, in 2002, by Europe.

But while so close, the world has not vanquished the virus. It remains endemic in six Asian and African nations, and on those continents, even after immunization initiatives finally halt transmission, polio will not disappear quickly given the millions afflicted.

Sever has witnessed the most pitiful on his repeated trips to assist the eradication effort. Many are street beggars who move their contorted bodies along on hands and knees. "The crawlers," they are called.

Though the disease and its menace have faded from this country's consciousness, the impact of how both were overcome continues to be felt. Salk's work was financed largely by individual Americans' response to the door-to-door fundraising of mothers and to the appeal of the winsome, scrubbed-fresh faces of polio's poster children. Its aftermath, however, ushered in a new era of government funding and surveillance of medical research. (The wholesale testing on hundreds of thousands of monkeys, on prisoners and on institutionalized children never would be permitted today.)

A host of other childhood vaccines soon followed, accompanied by shifting public expectations that immunizations should be made available to all regardless of income or race. Critics who decried socialized medicine were more or less quieted.

"Polio put all that in motion," Smithsonian curator Katherine Ott said.

For some Americans, there are personal reverberations. Those for whom the Salk vaccine came a few years or months too late, who lost family, friends or function because of the virus, say subsequent generations cannot understand the panic of that time. Polio was a scourge unlike any other.

"The difference is that everybody was at risk," explained Lauro Halstead, a physician who runs the post-polio program at National Rehabilitation Hospital. "Nobody was exempt."

Nobody, as in the 6-foot-4 Halstead, who in the summer of 1954 was an adventurous college student touring Europe. He probably contracted polio in France. He felt the first flush of fever while camping in Portugal and barely 24 hours later in Spain found he could hardly move his arms or legs.

As for Carol Vitiello Boyer, she awoke one night in 1952, took a step toward the bathroom and collapsed. The next thing the scared 3-year-old knew, she was in an ambulance rushing to the hospital. She would not return home to Fifth Street NE for six months, and when she did, shiny metal scaffolding supported her lower limbs.

Dolores Thompson suffered the simple misfortune of growing up on Virginia's Blue Ridge as the town of Wytheville reeled from the country's worst polio epidemic of 1950. Hers was labeled a "lighter case," but that was before the virus paralyzed her chest and sent her, at age 10, into an iron lung. Fifty-five years later, the submarine-like apparatus remains her lifeline.

"The only time I breathe without effort is when I'm in the iron lung," she said last week from her home in San Diego.

"No disease drew as much attention or struck with the same terror as polio," University of Texas historian David M. Oshinsky writes in a comprehensive new account, "Polio: An American Story." "And for good reason. Polio hit without warning. There was no way of telling who would get it and who would be spared. It killed some of its victims and marked others for life, leaving behind vivid reminders for all to see: wheelchairs, crutches, leg braces, breathing devices, deformed limbs."

'A Nasty, Nasty Disease'

The highly contagious virus wreaking this human destruction has been around since ancient times, passed person to person or through contaminated food or water. It replicates in the intestinal tract and, if contained there, causes the kind of mild symptoms that would mimic a head cold or a touch of flu. But when it breaches the intestinal wall and reaches the bloodstream, it invades the central nervous system, damaging or destroying the motor neurons and essentially disabling the muscles they activate.

Just a fraction of those infected suffer serious symptoms, much less paralysis. When they do, however, it happens quickly.

"A nasty, nasty disease," Virginia Hospital Center physician James Stallings said, thinking back to his medical school training in the early 1950s, when two students' pregnant wives came down with polio. Both delivered successfully -- some women actually gave birth in iron lungs -- but neither walked again. "I can't tell you how dramatic it was. These two lovely, young women and their babies. . . . It made you wonder what was going on in the world."

Such tragedies heightened public anxiety exponentially, as did the intensity that polio gained in the postwar years.

In 1946, 25,000 cases were reported. In 1952, nearly 58,000. The culprit, as epidemiologists later discovered, was the improved sanitation they'd actually expected to lower polio's incidence. Rather than being exposed to the virus at an early age, which helped trigger natural immunity, more and more children did not come into contact with the microbe until they were older, when its potential effect was more virulent.

"It was like a heavy, black shroud that hung over towns when summer came," Thompson said.

Wytheville was devastated by its encounter. The toll: 189 cases, nearly two dozen deaths. In one extended family, five of eight children fell ill. Incredibly, each survived.

A historical account, to be published next month by the town's museums department, recalls the highway signs warning of the outbreak; motorists sped through with their windows rolled up tight. The local semi-pro baseball team, the Statesmen, canceled its season because no opponents would come to play. Churches canceled services and broadcast Sunday school lessons over the radio. Everyone feared going out, making contact.

Among the few businesses that stayed busy were the two funeral homes, which turned their hearses into ambulances to transport the sick to Roanoke and Richmond. Some trips had so many patient-passengers that drivers would lay them crosswise in the vehicle. Sometimes the children didn't make it to the hospital. Even casket salesmen kept their distance.

"Unless you've lived through an epidemic like that, you have no idea what effect it had on the whole population of the town," said Ruth Anne Chitwood, whose husband and father-in-law worked nearly round-the-clock that summer as Wytheville's two family practitioners. "It was a wonderful day when the Salk vaccine was put on the market."

The number of polio cases fell quickly and precipitously. Adults joined children in the lines at immunization clinics. Those who didn't risked their health.

"We gave [the vaccine] to the kids, and I didn't take it. It was just stupid," recounted Walter Scheltema of Kensington, who in 1958 was named District of Columbia Polio Father of the Year for "tremendous moral courage in combating almost insurmountable obstacles." Several months earlier, a tremendous headache had leveled him one day at work. By morning, his legs were useless.

He was 26, with a 2-year-old daughter and baby boy.

Two details from those years stick in his mind. That April 12, 1955, holds import not just for the day's celebration but as the 10th anniversary of Roosevelt's death. And that the doctor at Providence Hospital who said Scheltema would forever be "a cripple," unable to get a job, was grossly wrong.

He first moved a little toe. After six months, he was discharged on crutches. As it did with any victim in need, the National Foundation for Infantile Paralysis, precursor to the March of Dimes, paid every dollar of his nearly $6,000 hospital bill.

He was intent on getting on with life, as were many polio victims, researchers say. "That doctor made me so mad," Scheltema, now 73, said recently. "I had a family, and I was going to take care of them."

Recurring Symptoms

Half a century later, the consequences of such gritty determination have come full circle for Scheltema and up to half of the estimated 1 million polio survivors in the United States. Even as they give Salk's triumph its due, polio's legacy is further altering their lives. Muscle weakness, fatigue and other disturbingly familiar problems have returned. The likely cause: years of overtaxing the muscles and nerves that carried on after others were ravaged.

Given the virus's eradication here -- and the "premature rush to judgment that polio is behind us," Halstead noted wryly -- many people have had difficulty accepting their symptoms.

The physician-researcher, one of the first to recognize the link between past and present, has graphed his own regression: Left arm, weaker. Walking endurance, less than a block. Halstead uses a motorized scooter for ever-shorter distances. He keeps modifying his movements for basic functions such as shaving and brushing his teeth. "I feel like I'm being sucked slowly into a bog," he said.

Boyer, who endured hip and leg surgeries into adolescence, has lost little since then. Now a disability rights specialist at the Department of Justice, she reflects on her experience with equanimity. Because of polio, she was a much-photographed poster child. She met movie stars and a vice president. In school, though, she always got picked last for dodge ball, and the other students mimicked her jerky gait.

"We're the end of the breed," she said. "The last of this breed of people who have this badge."

And forgotten too fast, Thompson fears. "Some people don't even know what polio is anymore." Her words are tinged with sadness. "It was a time in our history we can still learn from."

A diminutive woman who talks by necessity in an oddly modulated voice, she and her iron lung belong to the rarest subset of survivors. Over the years she figured out how to cope. She earned a college degree in English literature, proofread for magazines and wrote book reviews and poetry. "A very rewarding life," she said. For a time, she was strong enough to drive. No more.

She remembers the hospital doctor decades ago in Richmond who told her a cure was close and asked how she felt.

"I said I was happy." Lying there paralyzed, yet happy. "I didn't want any other child or adult to have to go through polio."

Staff researcher Bobbye Pratt contributed to this report.

An iron lung helps Dolores Thompson breathe. She has used the machine since polio paralyzed her at age 10 when she was living in Virginia.Jonas Salk became a national hero after his research team developed the polio vaccine.Carol Boyer was a poster child for polio research in the '50s. Lauro Halstead, a physician who runs a post-polio program at an area hospital, rides through Rock Creek Park with son Alex. Halstead contracted polio in 1954.In the 1950s, three of the five children in the Archer family of Wytheville, Va., were stricken by polio. The town had the country's worst epidemic of the disease in 1950, in which 189 cases were recorded and nearly two dozen people died. Dolores Thompson talks to her cat Dexter at her home in San Diego. The polio survivor recalls the anxiety over the disease in the 1940s and '50s. "It was a time in our history we can still learn from," she says.