Asthma is an immune-mediated inflammatory condition characterized by increased responsiveness to bronchoconstrictive stimuli. Viruses have been shown to play an important role in asthma, with viral infection being present during about 85% of exacerbations. However, the role they play in the onset of asthma is more controversial. Some respiratory viral infections might be protective, but there is a strong association between respiratory syncytial virus–induced bronchiolitis in infancy and recurrent wheeze up to 12 years of age. Both the respiratory tract and the immune system undergo rapid maturation during the first year of life, and it seems that postnatal development is affected by and affects responses to viral infections. Understanding postnatal developmental changes in the immune system might help to explain the origins and pathogenesis of asthma and thus the effectiveness or ineffectiveness of specific asthma therapies.