Respiratory tract infection refers to a number of infectious diseases involving the respiratory tract. An infection of this type is normally further classified as an Upper respiratory tract infection (URI or URTI) or a lower respiratory tract infection (LRI or LRTI). Lower respiratory infections, such as pneumonia, can cause more serious conditions than upper respiratory infections, such as the common cold. A variety of viruses cause different types of respiratory infections. Rhinovirus, coronavirus and adenovirus are the leading causes of the common cold. Influenza viruses infect the upper respiratory system and sometimes spread to the lungs causing pneumonia. Another virus called the respiratory syncytial virus (RSV) causes a respiratory infection called bronchiolitis in infants and toddlers. The symptoms of bronchiolitis include dry cough, rapid breathing and wheezing, a high-pitched sound sick children make when exhaling.