Protective and Harmful Effects of Viral Infections in Childhood on Wheezing Disorders and Asthma

It seems certain that environmental (not genetic) factors are responsible for the remarkable increase in prevalence of allergic respiratory diseases over the last 20 yr (1). One such factor might be improved hygiene, which has led to fewer childhood infections. However, it is well established that respiratory syncytial virus (RSV) infections cause many exacerbations in older children with asthma. In addition, many studies show that children with bronchiolitis are much more likely to suffer recurrent wheezing during the first 7-10 yr of life. It is not clear whether bronchiolitis reveals a preexisting weakness in the lungs, which later manifests as recurrent wheeze, or whether bronchiolitis itself causes persistent effects that could be avoided if RSV infections could be prevented (Figure 1). This is currently one of the most important unanswered questions in RSV research (2, 3). It seems that viral infections have both positive and negative influences on the development of airway inflammation, wheezing, and asthma. In this review, we summarize the evidence that viral infections influence the development of persistent airway inflammation, discuss some of the ways in which viral infections might affect the establishment of asthma, identify areas in which more information is needed, and try to define how this information could be obtained.