In Rocket Boys, how does Sherman defy the odds of his sickness?

Sherman had polio during his youth, which left his left leg shriveled and weak but he can still run better than Quentin. He never complains about his physical challenge and generally manages to keep up with the rest of the boys. Sherman is in charge of publicity and setting up the rocket range for the BCMA. Sherman is in Sonny's words "brave and good" and, during the time when Sonny is numb, after Mr. Bykovski's death, he tells Sonny he must go to the bus station and say he is sorry to Mrs. Bykovski. Because Sonny does so, Mrs. Bykovski says that there is one thing that Sonny can do for her and her dead husband... to keep building rockets. Sherman goes to college and becomes an engineer after high school. He dies unexpectedly of a heart attack when he is twenty six years old.