Viral infections and asthma inception.

Respiratory tract infections caused by viruses have been implicated in the pathogenesis of asthma. Of these respiratory pathogens, viruses have been demonstrated to be associated with asthma epidemiologically in at least 3 ways ( Fig 1 ). First, during infancy, certain viruses have been implicated in the inception of the asthmatic phenotype. Genetic susceptibility, particularly genes coding for atopic phenotypic characteristics, might differentiate, at least in part, those children who are destined to have persistent wheezing, asthma, or both later in childhood. Second, repeated exposure to infectious viruses in daycare centers or in households with multiple older siblings increases the number of respiratory infections, but in doing so, it might paradoxically reduce the long-term risk of allergies and asthma through either pre-existing or newly formed alterations in cytokine response profiles. Third, in patients with established asthma, particularly children, viral upper respiratory tract infections play a significant role in producing acute exacerbations of airway obstruction that might result in frequent outpatient visits or in hospitalizations. This review will highlight available data on respiratory syncytial virus infections and their relationship to asthma inception in childhood.