How FDR caught polio?

Did FDR really catch polio from having swum in unhygienic conditions as suggested in the film? I'm trying to figure out if his polio was avoidable or not. Perhaps if he had opted for another activity one day he would have walked through life and the Warm Springs clinic would not have saved so many lives and the Salk and Sabin vaccines would not have been developed in the 1950s. Does anyone know for certain?