<P> Infections can affect any part of the respiratory system . They are traditionally divided into upper respiratory tract infections and lower respiratory tract infections . </P> <P> The most common upper respiratory tract infection is the common cold . However, infections of specific organs of the upper respiratory tract such as sinusitis, tonsillitis, otitis media, pharyngitis and laryngitis are also considered upper respiratory tract infections . </P> <P> The most common lower respiratory tract infection is pneumonia, an infection of the lungs which is usually caused by bacteria, particularly Streptococcus pneumoniae in Western countries . Worldwide, tuberculosis is an important cause of pneumonia . Other pathogens such as viruses and fungi can cause pneumonia for example severe acute respiratory syndrome and pneumocystis pneumonia . A pneumonia may develop complications such as a lung abscess, a round cavity in the lung caused by the infection, or may spread to the pleural cavity . </P> <P> Poor oral care may be a contributing factor to lower respiratory disease . New research suggests bacteria from gum disease travel through airways and into the lungs . </P>

What are three main disorders of respiratory system