[Infection and asthma]

Infection is frequent in all stages and forms of asthma. It is a source of exacerbations of all degrees of severity. Bacterial infection, especially with commensal organisms from the upper respiratory tract, may cause bronchial hyperreactivity (HRB) by various mechanisms, specific--microbial delayed hypersensitivity, probably rate; IgE-dependent hypersensitivity others non-specific: bronchial inflammation, source of mediators of bronchial constriction; activation of complement, direct histamine liberation; beta blockage...; or mixed mechanisms. Respiratory viral infection probably plays the most important role in the natural history of asthmatic disease, especially in infants and young children by inducing or amplifying HRB and creating a transitory obstructive syndrome of the small airways. This virus-induced HRB may develop into a respiratory syncytial virus viraemia at first or remain latent found by tests of bronchial activity. The pathogenic mechanisms of this HRB are often multi-factorial and interlinked: they are chiefly linked to the various cytopathic effects of the virus on the respiratory epithelium, to the viral inflammation; but also to disturbance of the equilibrium of the autonomous nervous system (beta blockage, excitation of the cholinergic receptors); finally in some areas, to virus-induced amplification of the local and systemic immune responses by IgE that favours clinical consideration of atopy.