Links between viruses and respiratory allergy are not easy to understand in children. For example, the risk of atopy or asthma is increased after an infection with syncytial respiratory virus. In some studies, more the child suffers from viral infections, more the risk of atopy increases. On the other hand, other studies state that the development of allergy is reduced if a child enters day nursery before 12 months, and consequently if he is exposed early in life to viruses. Measles and hepatitis A viruses could also protect from allergy. In fact, viruses seem modulate the expression of a preexisting atopic status. Depending on their nature and their circumstances of occurrence, they could induce some sensitizations or inversely protect from atopy by facilitating the lymphocyte Th2 or Th1 response.