Acute respiratory diseases account for an estimated 75% of all acute morbidities in developing countries, and most of these are caused by viruses. Upper respiratory tract infections (URTI) are among the most common infections in children, occurring 3--8 times per year in children under 5 years of age, and often causing acute asthma exacerbations or acute middle ear infections. The U.S. Centers for Disease Control and Prevention's National Vital Statistics Report indicates that there are 12–32 million episodes of URTI each year in children under 2 years of age. Viral respiratory tract infections can be caused by 18 different types of viruses. These include conventional agents such as influenza A and B, respiratory syncytial virus (RSV) types A and B, parainfluenza virus (types 1–4), adenovirus (50 types), the “common cold” viruses (including 2 coronaviruses and over 100 types of rhinovirus), and 6 viruses discovered since 2001—metapneumovirus, 3 coronaviruses (including severe acute respiratory syndrome (SARS)– associated virus), bocavirus, and avian influenza virus (H5N1).