Viral infections commonly affect the upper or lower respiratory tract. Although respiratory infections can be classified by the causative virus (eg, influenza ), they are generally classified clinically according to syndrome (eg, the common cold , bronchiolitis , croup , pneumonia ). Specific pathogens commonly cause characteristic clinical manifestations (eg, rhinovirus typically causes the common cold, respiratory syncytial virus [RSV] typically causes bronchiolitis), and each pathogen can also cause many of the general symptoms of viral respiratory syndromes.