Nicolas Stettler, M.D., M.S.C.E., is assistant professor of pediatric epidemiology at the Children's Hospital of Philadelphia specializing in gastroenterology, hepatology, and nutrition. A graduate of Lausanne University Medical School in Switzerland, Stettler holds an M.S. in clinical epidemiology from the University of Pennsylvania. He is board-certified in pediatrics and nutrition. Susan Shelly is a writer and researcher based in Shillington, Pennsylvania. She is a coauthor of The Encyclopedia of Blindness and Vision impearment Second Edition in the Facts On File Library of Health and Living series.