Risk factors include infants <6 months of age, premature infants born <35 weeks' gestation, infants born with lung disease, infants <2 years of age with heart disease, infants with school-aged siblings, infants who attend day care, family history of asthma, regular exposure to secondhand smoke or air pollution, multiple birth babies, peak RSV season (fall to end of spring), being male, immunocompromised patients (eg, severe combined immunodeficiency, leukemia, or undergoing organ transplant), <1 month or no breast-feeding, and others sharing the bedroom with the infant. High altitude increases the risk of RSV hospitalization. Risk factors for more severe or fatal RSV disease include a premature infant, an infant with complex congenital heart disease (CHD), especially those that cause cyanosis or pulmonary hypertension; an infant with bronchopulmonary dysplasia (chronic lung disease [CLD]); and an infant with immunodeficiency with lymphopenia or receiving therapy that causes immune suppression.