ABSTRACT
ABSTRACT Viral respiratory tract infections (VRTI) have a strong epidemiological relationship with wheezing disorders. At least three different types of relationships between VRTI and wheezing disorders have been described. First, VRTI during early childhood can induce recurrent wheezing. For example, respiratory syncytial virus (RSV) infection is followed by post-bronchiolitis wheezing in about 50% of cases. Pre-existent diminished lung function is probably an important mechanism of virus-induced wheezing. It is debated whether viral infections can change immunological status or induce allergy. Second, viral infections are the most important triggers of asthma exacerbations in atopic patients. In 85% of asthma exacerbations in children VRTI are found in airways. In the majority of cases rhinovirus is the infectious agent. Third, exposure to respiratory viruses during infancy might prevent allergy development. The mechanism of virus-induced prevention of asthma is by skewing the immune system towards a Thl-phenotype. Different types of relationships between VRTI and wheezing can co-exist in one individual. For example, common colds during infancy may induce both increased risk of recurrent wheezing during childhood and decreased risk of allergic asthma later in life. The temporal relationship between viral infection and allergy development, as well as the interplay between genetic factors and viral infection, appear the keys to understand the intriguing relationship between viruses and asthma.
