Viruses are responsible for the majority of respiratory infections in childhood,causing considerable morbidity and mortality. It is estimated that in the United States approximately $ 652 million per year is spent on medical costs for respiratory syncytial virus (RSV) related disease alone (Paramore et al., 2004). Viruses cause a variety of respiratory diseases in children from the common cold to life-threatening pneumonia and bronchiolitis. The host reacts to a viral infection with a combination of innate and adaptive immune mechanisms, usually resulting in the clearance of the virus and clinical recovery. However, there is an accumulating evidence for a number of viral infections that the host immune response actually enhances disease in the course of clearing virus from the infected organs. Interestingly, the effectiveness of the immune response seems to be dependent on the age and probably genetic background of the child. This has important implications for treatment as well as vaccine development.