Abstract
Recent studies of the natural history of asthma have shifted attention toward viral respiratory tract illness in early life as a major risk factor associated with the development of the most persistent forms of the disease. Although early aeroallergen sensitization is strongly associated with chronic asthma, several trials in which single-aeroallergen exposure in pregnancy and early childhood was successfully accomplished and compared with sham avoidance have failed to show any decrease in asthma incidence. New evidence suggests that complex interactions occur between viral infection and aeroallergen sensitization in genetically susceptible subjects that trigger the immune responses and airway changes that are characteristic of persistent asthma. The finding that exposure to bacterial products among children raised on farms is associated with diminished asthma prevalence during the school years has now been replicated, and experimental studies have suggested that these effects are mediated by the activation of regulatory T cells in the airway. It is thus plausible to hypothesize that primary prevention of asthma could be attained through surrogate therapeutic interventions that activate similar mechanisms in young children at high risk for asthma.