Design,Setting,andParticipants Prospectiveobservationalandcase-controlstudy during 2007 in a rural Kenyan district hospital. Participants were children aged 1 day to 12 years, residing in a systematically enumerated catchment area, and who either were admitted to Kilifi District Hospital meeting World Health Organization clinical criteria for severe pneumonia or very severe pneumonia; (2) presented with mild upperrespiratory tractinfectionbutwerenotadmitted;or(3)werewellinfantsandchildren attending for immunization