This is an epidemiology study designed to implement hospital-based active surveillance for intussusception (IS) in children <1 year of age and lower respiratory tract infection (LRTI)-related deaths among children between 29 days and 1 year of age in Mexico. An existing linked database of the "Instituto Mexicano del Seguro Social (IMSS)" will be used. The study described in this protocol is a pilot study, which will evaluate the methodology to be used in a later Post-Marketing Surveillance (PMS) study.