PAUL A. OFFIT, M.D., is Chief of Infectious Diseases at the Children s Hospital of Philadelphia and the Henle Professor of Immunologic and Infectious Diseases at the University of Pennsylvania School of Medicine. He and Louis Bell are the authors of What Every Parent Should Know about Vaccines. BONNIE FASS-OFFIT, M.D., a pediatrician in private practice, is CoMedical Director of Kids First-Haverford/Radnor. LOUIS M. BELL, M.D., is Medical Director of the Hospital Infection Control Department and an attending physician in infectious diseases and emergency medicine at the Children s Hospital of Philadelphia.