Viral infections and allergies.

Respiratory viral infections have been implicated in the origin of, protection from and exacerbation of allergy-related symptoms in a variety of ways. Viral infections are closely linked to infantile wheezing. Severe bronchiolitis in early infancy may predispose to chronic childhood asthma as well as allergic sensitization; alternatively it could represent a marker of susceptible individuals. In contrast, repeated mild infections in early life may have a protective role in the development of asthma or atopy by driving the immune system towards Th1 responses. However, evidence on this hypothesis is not consistent as far as respiratory viruses are concerned. Several factors, including the presence of an atopic environment, timing of exposure and severity of the infection, interactively contribute to the allergy-infection relationship. In the present report, recent data on the role of viral infections in the development and progression of allergy and asthma are reviewed.