Viruses are the leading causes of acute lower respiratory tract infection in most populations. Common viral respiratory infections can be categorized by site of anatomic involvement (e.g., alveolar spaces, bronchioles, trachea, conjunctiva, middle-ear spaces), disease syndrome (e.g., rhinitis, pharyngitis, bronchiolitis, pneumonia), or etiologic agent (e.g., respiratory syncytial virus [RSV], human metapneumovirus, parainfluenza virus [PIV], adenovirus). See Chap. 59: Sore Throat, Earache, and Upper Respiratory Symptoms for additional details on viral respiratory infections.