Viral and bacterial infections in the development and progression of asthma

Viral respiratory infections produce wheezing illnesses in patients of all ages, In infancy, infections with respiratory syncytial virus and parainflu enza virus are the major cause of bronchiolitis and croup, whereas infectio ns with common cold viruses such as rhinoviruses are the principal triggers for wheezing in older children and adults with asthma. In addition to caus ing increased wheezing in asthma, there is mounting evidence that infection s early in childhood can affect the development of the immune system and th ereby modify the risk for the subsequent development of allergies and asthm a, Both of these effects appear to be mediated by virus-induced immune resp onses, Early during the course of viral infection, resident cells in the ai rway are activated in an antigen-independent fashion, triggering antiviral responses but also activating and recruiting cells to the airway that could contribute to airway obstruction and respiratory symptoms, Virus-specific T- and B-cell responses may also have dual effects in the presence of preex isting airway inflammation. Finally, there is evidence of synergistic inter actions between allergen- and virus-induced airway inflammation. It is Like ly that greater definition of mechanisms of virus-induced inflammation will provide therapeutic targets for the treatment and possibly the prevention of allergies and asthma.