Most of the data on the pathogenesis of asthma is based on information obtained through bronchial biopsies and bronchoalveolar lavage in adults and young adults. Ethical considerations linked to the invasive nature of airway endoscopy have limited the studies on the pathophysiology of asthma in infancy and early childhood. Although there is evidence that an asthma-like inflammation, with increased inflammatory cells and thickening of the lung basement membrane, may be present also at a very early age, clinical and epidemiologic studies suggest that asthma manifestations in preschool children may significantly differ from those observed in older subjects. In western countries, the vast majority of infants and young children has episodic (or intermittent) asthma, and the exacerbations generally defined "wheezing episodes" occur more frequently with a seasonal pattern being usually related to acute viral infections. There is strong epidemiological evidence that approximately 2/3 of all children who wheeze because of viral infections in early life (and are not atopic) have a transient condition that tends to disappear during early school years. All respiratory viruses may be implicated in the wheezing episodes, the principal being respiratory syncytial virus (RSV) and, with a lower frequency, adenovirus and parainfluenza viruses during the first 3 years of life, and rhinoviruses after that age. Infants and preschool children have on average 6-8 "colds" per year, but the illness tends to be limited to the upper respiratory tract alone in a considerable proportion of individuals, without causing symptomatic involvement of the lower respiratory tract. The variety of factors determining the different outcomes are only partially known, but complex interactions between the intrinsic pathogenicity of the virus and host factors, including the socio-economic conditions of the family, are central to define the type of manifestations and the severity of the process.