All data will be collected at enrollment and at each follow-up visit on a standardized data sheet. Baseline demographic data( age, gender, address, ethnicity, household size, income, parent education, caregivers, household appliances) and medical information( birth history, breastfeeding and weaning, growth, immunization, smoke exposure, co-morbidity, daily hospitalization information) will be obtained from parents and medical charts. CXRs and routine blood test results will also be recorded.