Routine childhood immunizations and adult boosters prevent the disease. See: Diphtheria immunization (vaccine)
American Academy of Pediatrics Committee on Infectious Diseases. Recommended immunization schedules for children and adolescents--United States, 2008. Pediatrics. 2008 Jan;121(1):219-20.
Advisory Committee on Immunization Practices. Recommended adult immunization schedule: United States, October 2007-September 2008. Ann Intern Med. 2007 Nov 20;147(10):725-9.