On the morning of Aug. 11, 1921, Franklin D. Roosevelt got out of bed and tried to make it to the bathroom in the summer house on Campobello Island, off the coast of Maine, where he and his family had vacationed since he was a child.

He had felt sick the day before. After hours of sailing and swimming, his legs began to ache. He developed an uncontrollable shiver and had gone to bed early. Now, he was worse. He had a fever of 102, and as he struggled across the hall, his left leg gave way beneath him.

It was a transformative moment in American history, and the start of the “central event” in the life of one of the country’s greatest presidents, historian Hugh Gregory Gallagher has written.

On that morning 100 years ago this month, the man who would lead the United States through the Great Depression and World War II and expand the way government helped people, was experiencing the first crippling symptoms of polio.

He was 39, and the disease would soon place him in a wheelchair for most of the rest of his life.

It would alter his emotional makeup, Gallagher has written. It changed the way he viewed the world. And it would launch the exquisite political theater in which FDR engaged so famously on the public stage.

His rumpled fedora; his jaunty cigarette holder; his pince-nez glasses; his Naval cape; and the steel leg braces that let him stand and appear to walk on his own.

“The complete package of props,” Gallagher wrote.

“Together with the … tilt of the head, the wave of the hand …[and] the smile, made [him] seem to the American people … as close as a family member,” Gallagher wrote in his 1985 book, “FDR’s Splendid Deception.”

And although he worked hard to obscure the details of his disability, he was, in part, defined by it, admired and remembered for it. An element of his presidential memorial amid the cherry blossoms in Washington depicts him sitting in a special wheelchair he designed himself.

Paul Sparrow, director of the Franklin D. Roosevelt Presidential Library and Museum, in Hyde Park, N.Y., said: “I think polio changed FDR. I think polio changed America. I think FDR’s reaction to polio changed the world.”

But most of that was in the future.

On a Thursday morning in 1921, no one knew what was happening to him.

Polio at the time was also called infantile paralysis because it seemed to infect mostly children.

The disease is caused by a contagious virus that can lead to severe disability and death, according to the National Institutes of Health. It is ingested orally and, in the worst cases, enters the central nervous system and destroys motor neurons, causing paralysis of limbs.

Many of those who survived serious infection had to wear painful metal leg braces. There would be no vaccine for 30 years.

Exactly five summers earlier, in 1916, there had been a terrible polio epidemic along the East Coast, where 27,000 people were infected.

It was especially bad in New York City where there were 19,000 cases and almost 2,500 deaths, according to Roosevelt biographer Geoffrey C. Ward.

During a week in early August, there were 1,151 cases and 301 deaths, historian Tony Gould wrote in his 1995 book, “A Summer Plague, Polio and its Survivors.”

Tens of thousands of the children of the wealthy were shipped out of town to avoid infection. Cats and dogs were suspected carriers and thousands of strays were gathered up and killed.

Guards were placed at the entrances to towns outside New York City to block the arrival of fleeing city dwellers, Gould wrote.

That summer, the Roosevelt family was vacationing as usual on Campobello Island, where FDR’s wealthy father had first taken him in 1883.

In 1916, FDR was then serving as assistant secretary of the Navy and had slipped out of Washington for a brief visit in late July.

During the visit, according to Ward’s biography, he noticed an unusual number of flies in the house. Fearful that they might be polio carriers, after he left, he wrote to his wife, Eleanor, who had stayed behind with their children:

“The infantile paralysis in N.Y. and vicinity is appalling. Please kill all the flies … I think it really important.”

In August, he told his wife that he was afraid to bring the family home from vacation by train:

“There is much I.P. [infantile paralysis] in Boston, Springfield, Worcester … Also the various villages are keeping motorists with children out.”

In late September, he was able to borrow the Navy secretary’s official yacht, the Dolphin, and transport the family to his mother’s home in Hyde Park by water.

That year, the Roosevelts escaped polio.

By 1921, the epidemic had waned. But the virus still lurked.

On July 27, Roosevelt was part of a delegation of VIPs that visited an encampment of 2,100 Boy Scouts from New York City near Bear Mountain, about 40 miles north of the city.

FDR was then, among other things, president of the Boy Scout Foundation of Greater New York. He had been the Democrats’ vice-presidential candidate on a ticket headed by Ohio governor James M. Cox the year before. But the Republicans had won the election in a landslide.

“There were parades and speeches,” Ward wrote of the encampment. FDR posed for pictures with crowds of scouts, and was toastmaster at a fried chicken dinner before returning to the city that evening.

“With him went a mysterious virus … perhaps … ingested at some point during the hot, hectic day … already moving through his bloodstream,” Ward wrote.

Two weeks later on Campobello Island, it began showing its effect on his body.

He had managed to get to the bathroom, shave and hobble back to bed. He was feverish and had a severe pain in his back.

“I tried to persuade myself that the trouble with my leg was muscular, that it would disappear as I used it,” he recalled later. “But presently it refused to work, and then the other.”

Roosevelt was 6-feet, 3-inches tall. The next day he could no longer stand by himself.

Doctors were called. One thought he had a bad cold. Another thought he had a blood clot.

“I don’t know what’s the matter with me,” Roosevelt told his friend and adviser, Louis Howe, who sat by his bed. “I just don’t know.”

Howe wrote to an uncle of FDR’s in New York, described the symptoms, and asked him to consult any experts he knew.

Word came back that Roosevelt might have polio.

The country’s leading authority on the disease, Robert Williamson Lovett, of Boston, was summoned.

On Aug. 25, he reached Campobello and examined FDR. Roosevelt was mostly paralyzed from the waist down. The diagnosis was “perfectly clear,” the doctor said.

The young politician’s bright future was now darkened by polio.

On a windy and overcast Sunday afternoon eight years later, a New York teenager, Philip Hamburger, went to see a speech in Manhattan by the newly-inaugurated governor of New York, according to Gallagher’s book.

The governor, a Democrat, had won by a slim 25,000 votes, despite a nationwide Republican avalanche.

He had spent years overcoming an enormous physical handicap. He had fought his way back onto the political stage. And he had campaigned for the office relentlessly — making seven speeches in seven towns in one day.

His name was Franklin Roosevelt. In the years ahead, he would be elected president four times, leading the country through economic upheaval, political change and global war before dying in office in 1945.

But the small event Hamburger witnessed seemed to capture an essence of Roosevelt’s connection to the people.

Later a journalist with the New Yorker, Hamburger recalled that FDR spoke that day in 1929 sitting at a desk in front of the podium of a small community hall. Eleanor Roosevelt sat beside him.

Hamburger didn’t recall much about the speech. But he remembered how the Roosevelts made their exit.

The only way out of the hall was down the center aisle and through the crowd. The distance was only about 100 feet. But what happened stayed with Hamburger for many years.

Roosevelt wore heavy leg braces and walked with two canes.

“First one foot and one cane came forward, then the other,” Hamburger wrote in 1962.

The pace was agonizing, and Eleanor was careful to stay alongside her husband.

As they proceeded, she thanked people for coming.

“The audience, as though hypnotized, did not leave,” Hamburger recalled. “It stood and watched.”

“The governor was intent upon the task before him: to reach the street and the sanctuary of his limousine without help,” he wrote. “Mrs. Roosevelt seemed to sense that we knew we should not stay, but we could not leave.”

Followed by the crowd, the couple finally reached the street, and FDR was helped into the car. His wife joined him.

“Mrs. Roosevelt opened a window and waved,” Hamburger wrote. “An audience of strangers had become a group of friends.”

“Goodbye” she called. “Goodbye.”

This story has been updated.