[The interactions between microorganisms and the small airways. A paediatric focus].

The spectrum of respiratory viruses is expanding and emerging diseases have been described regularly over the last fifteen years. The origin of these emerging respiratory viruses may be zoonotic (by crossing species barrier, after changes to RNA viruses such as avian influenza virus type A or coronaviruses), or related to the use of new identification techniques (metapneumovirus, bocavirus). The relationship between bronchiolitis and asthma is now better understood thanks to prospective follow up of birth cohorts. The role of rhinovirus has become predominant with respect to respiratory syncytial virus. The identification of predisposing factors immunological, functional, atopic and genetic, for the onset of asthma after rhinovirus infection suggests that viral infection reveals a predisposition rather than itself being a cause of asthma. The role of bacteria in the natural history of asthma is also beginning to be better understood. The results of the COPSAC Danish cohort have shown the frequency of bacterial identification during wheezy episodes before 3 years, and the impact of bacterial colonization at the age of one month on the onset of asthma by age 5 years. The role of bacterial infections in severe asthma in young children is also discussed.