1. Division of Pediatric Gastroenterology and Nutrition, Massachusetts General Hospital, Charlestown, Massachusetts 02129-4404;
2. Departamento de Infectologia, Instituto Nacional de Ciencias Medicas y Nutricion, Mexico City DF 14000, Mexico;
3. Center for Epidemiology and Biostatistics, Cincinnati Children's Hospital Medical Center, Cincinnati, Ohio 45229-3039;