Infections. During infancy, a number of viruses have been associated with the inception of the asthmatic phenotype. Respiratory syncytial virus (RSV) and parainfluenza virus produce a pattern of symptoms including bronchiolitis that parallel many features of childhood asthma53,54. A number of long-term prospective studies of children admitted to the hospital with documented RSV have shown that approximately 40% will continue to wheeze or have asthma into later childhood53. On the other hand, evidence also indicates that certain respiratory infections early in life, including measles and sometimes even RSV, may protect against the development of asthma55,56. The data do not allow specific conclusions to be drawn. The “hygiene hypothesis” of asthma suggests that exposure to.
