Wheezing is a common complication of respiratory infections (Table 1). In infancy, infections with respiratory viruses, particularly respiratory syncytial virus (RSV) and parainfluenza virus (PIV), cause wheezing and bronchiolitis, which can be recurrent. Moreover, infants with reduced lung function and/or exposure to tobacco smoke are at greatest risk for developing wheezing with RSV infection (1), but only a subset of those who wheeze in infancy subsequently develop persistent asthma. For those who develop persistent wheezing, atopy, as indicated by elevated total immunoglobin E (IgE) and the development of allergen-specific IgE, is a major risk factor in this process (2).

There is now evidence that viral infections in early childhood may also act on the immune system to modify the subsequent risk of allergen sensitization and/or asthma (3). For example, several studies have shown that the odds of allergen sensitization are inversely related to the number of older siblings in the family, which presumably determines the exposures to infectious diseases in early childhood (4–6). In addition, data from Africa indicate that measles infection in early childhood reduces the risk of allergen sensitization (7). Some bacterial infections may have similar effects: Japanese school-