Respiratory syncytial virus (RSV) is the principal cause of bronchiolitis and pneumonia in infants and young children worldwide. Deficits in cellular immunity appear to promote severe RSV disease in children with malignancies, those undergoing chemotherapy and bone marrow transplant recipients. Respiratory syncytial virus infection appears to exacerbate pulmonary symptoms of cystic fibrosis. In such patients RSV disease may result in a prolonged hospital course, which is often complicated by the need for mechanical ventilation. Retrospective analyses of hospital admissions for RSV bronchiolitis among Native American and Alaskan Native children younger than 1 year of age have demonstrated rates of 62 per 1000 or higher, compared with the national average of 34 per 1000. Among these ethnic groups, specific host factors as well as environmental factors appear to contribute to these comparatively high rates of hospitalization for RSV infection. Respiratory syncytial virus has the potential to cause disease in all age groups. A 3-year observational study found that individuals who lived in a community setting, or who cared for young children on a consistent basis, experienced acute respiratory infections more commonly than those living independently or whose interaction with children was limited.