Upper respiratory tract infections and wheezing illnesses occur frequently in early childhood. Most viral infections associated with wheeze in infancy are attributable to respiratory syncitial virus, whereas in older children rhinovirus, influenza and parainfluenza virus prevail. The detection rate of viruses has increased since PCR techniques have been used. A British study for example showed that in 80% of episodes in asthmatic children aged 9-11 years viruses could be detected in nasal aspirates which had been taken within 4 days of the wheezing attack. In most cases rhinovirus was identified. Two major hypotheses have been proposed to explain the association between respiratory tract infections and subsequent respiratory abnormalities. One hypothesis states that viral infections early in life damage the growing lung or alter host immune regulation. The second hypothesis holds that respiratory infections are more severe in infants and children with some underlying predisposition. These notions await further clarification. Early childhood bacterial and viral infections may, however, also be associated with a reduced risk of developing atopic sensitization or allergic conditions, as the results of several recent studies suggest.