Americans remember Franklin Delano Roosevelt as the president who led the country through the Great Depression and World War II. He bolstered the nation's spirits with his confidence, strength and optimism, despite being crippled by polio, a disability that's largely invisible in photographs and newsreels of his presidency.

But historian James Tobin says, despite misimpressions to the contrary, Americans of Roosevelt's day were well-aware of his disability. In fact, Tobin says, Roosevelt's struggle to overcome his affliction was an important part of the personal narrative that fueled his political career.

Tobin tells Fresh Air's Dave Davies, "[Roosevelt] only discovered who he really was through the ordeal of polio. ... It gave him a kind of confidence in his own strength that perhaps no one can have until you're tested."

Roosevelt contracted polio at the age of 39, and Tobin's new book explores his battle with the illness and the ways it molded his character and influenced his rise in the Democratic Party. Tobin has written previous books about the Wright brothers and war correspondent Ernie Pyle. His new book is The Man He Became: How FDR Defied Polio to Win the Presidency.

On how good sanitation made kids more susceptible to polio

Before the polio vaccine, pretty much every little kid ingested the polio virus but was protected by maternal antibodies, so even though the virus passed through his or her system, they wouldn't become sick with disease. As sanitation got better, they had fewer immunities, and so if the virus did creep into a community with good sanitation, kids were more likely to get sick and to become seriously ill.

Roosevelt had grown up on an isolated estate in upstate New York. He probably had immune deficiencies to begin with — he was always getting sick with one bug or another. So he was particularly susceptible when, even though he was an adult, he contracted the virus.

On FDR's recovery efforts

Roosevelt went into a long period of physical rehabilitation after recuperating for several months. By December 1921, he was ready to have a physical therapist begin to massage his muscles, begin to work his muscles, begin to try to figure out exactly the extent of the damage. As more and more time passed in the coming weeks and the early months of 1922, he was able to begin to exercise on his own. This was laborious, difficult: He really could not even stand up on his own at all for months and months, and so this was a matter of lying in his bed, performing these minute little exercises, trying to move one muscle and then another muscle. ... It was painstaking, it was difficult. He had to have his legs put into casts at one point to prevent against contractures. ... It was really a grueling process.

On misimpressions of FDR's openness about his condition

When I've talked to people in the past ... I've always asked them, "Did you know about FDR's condition?" And they've always said yes. What they say is, "We realize later that he was more disabled than we knew, but we certainly knew he was disabled, we knew that he couldn't walk." I think that this misimpression comes from a couple of things:

There was a book published in the 1980s called FDR's Splendid Deception in which the writer, Hugh Gregory Gallagher, I think overstated the evidence for FDR covering this up. And then in the debate over the Roosevelt memorial in Washington that took place in the 1990s, that theme got repeated over and over again by various advocates in that argument. And then it got put into a couple of television documentaries, and so it just had a viral effect.

All you have to do is go back to the newspapers of the time, especially from the 1920s when Roosevelt was making his political comeback, and his disability was discussed constantly. He was very frank about it. So there's no question that people knew about it. And you see during his presidency, people who were themselves disabled, people who had polio, their children had polio, writing to FDR in the White House by the hundreds and talking about his disability. The March of Dimes [nonprofit] itself, which came about during Roosevelt's presidency, he was the leader of it, was an effort to fight polio. The polio campaign that was waged every year had Roosevelt as its figurehead.