Respiratory tract infection refers to any of 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 a lower respiratory tract infection (LRI). Lower respiratory infections, such as pneumonia, tend to be far more serious conditions than upper respiratory infections, such as the common cold. URIs represents the most common acute illness evaluated in the outpatient setting and is a nonspecific term used to describe acute infections involving the nose, paranasal sinuses, pharynx, larynx, trachea and bronchi. URIs range from the common cold - typically a mild, self-limited, catarrhal syndrome of the nasopharynx - to life-threatening illnesses such as epiglottitis. Symptoms of URIs can include cough, sore throat, runny nose, nasal congestion, headache, low-grade fever, facial pressure and sneezing. Influenza is a systemic illness that involves the upper respiratory tract and should be differentiated from other URIs. LRIs are generally more serious than URIs. LRIs are the leading cause of death among all infectious diseases. The two most common LRIs are bronchitis and pneumonia. Influenza affects both the upper and lower respiratory tracts, but more dangerous strains such as the highly pernicious H5N1 tends to bind to receptors deep in the lungs. Viruses cause most URIs, with rhinovirus, parainfluenza virus, coronavirus, adenovirus, respiratory syncytial virus, coxsackievirus and influenza virus. Human metapneumovirus is a newly discovered agent causing URIs. Group A beta-hemolytic streptococci (GABHS) cause 5% to 10% of cases of pharyngitis in adults. Other less common causes of bacterial pharyngitis include group C beta-hemolytic streptococci, Corynebacterium diphtheriae, Neisseria gonorrhoeae, Arcanobacterium haemolyticum, Chlamydia pneumoniae, Mycoplasma pneumoniae and Herpes simplex virus. Streptococcus pneumoniae, Haemophilus influenzae and Moraxella catarrhalis are the most common organisms that cause the bacterial superinfection of viral acute sinusitis. Less than 10% of cases of acute tracheobronchitis are caused by Bordetella pertussis, B. parapertussis, M. pneumoniae or C. pneumoniae.