Childhood asthma and infection: virus-induced exacerbations as determinants and modifiers.

Respiratory infections have been implicated in the origin and exacerbation of asthma in a variety of ways; however, systemisation of this knowledge in a way helpful for disease management remains suboptimal. Several conceptual issues need to be taken into account: the fact that the effects of an infection may vary according to genetic background, the current immune status of the host, and parallel environmental stimuli, in addition to the particular infectious agent itself. Moreover, childhood is a very special period because of the continuous processes taking place, such as neural, immune and respiratory maturation. Epidemiological studies have convincingly demonstrated that the majority of asthma exacerbations, in both adults and children, follow viral upper respiratory tract infections. Asthma exacerbations are still often unresponsive to current asthma treatment, and new therapeutic approaches are required. This review presents current knowledge on the associations between infection and exacerbation of established asthma with respect to definitions, epidemiology, mechanisms and treatment.