Disorders that affect the respiratory system include diseases of the nose, mouth, throat, larynx, trachea, bronchial tubes,
and lungs. These diseases may result from a variety of causes and can be acute or chronic.
INFECTIONS Infections are the most common cause of respiratory disease and may be produced by viruses, bacteria, fungi, and protozoa.
Viruses
Viruses are the most common cause of respiratory tract infections. They are highly communicable from person to person and cause the common head cold (rhinitis) and chest cold (tracheobronchitis; see cold, common). Some viruses cause
Infection of the lung itself, as in pneumonia. Common viral illnesses, such as different types of influenza, measles, polio, and chicken pox, may be associated with respiratory tract complications.
Bacteria
Many bacteria can infect the respiratory system. The most common infectious bacteria are species of Streptococcus, Staphylococcus, and Haemophilus, which cause such diseases as sinusitis, bronchitis, pharyngitis, strep throat, tonsillitis, whooping cough, and pneumonia. The usual treatment for these infections is antibiotics. Bacterial pneumonia, a serious infection, continues to be a frequent cause of mortality, especially in elderly people.
Mycoplasmas and Legionellas are tiny bacteria-like organisms that cause sinusitis, pharyngitis, bronchitis, and pneumonia. Legionnaires' disease, a potentially life-threatening pneumonia caused by Legionella pneumophilia, is usually acquired from contaminated water in air-conditioning systems.
The bacteria Mycobacterium tuberculosis causes tuberculosis (TB), which affects the lung primarily but may spread to other parts of the body. TB is readily communicated from one person to another via small droplets that are coughed into the atmosphere and can be inhaled by others. TB had been decreasing in frequency since the 1940s, but began to show a significant resurgence in the late 1980s, with the advent of HIV infection (see AIDS), as individuals with weak immune systems are particularly susceptible to TB infection.
Fungi
Fungi that are common in soil and water can be inhaled and infect the sinuses, airways, and the lungs themselves. Histoplasmosis, coccidiodomycosis, and blastomycosis, are among the diseases caused by fungi. Fungal infections of the respiratory tract are also common in patients with weaker or impaired immune systemsÑfor example, cancer patients who are undergoing extensive chemotherapy, people with AIDS, and the very young or very old. Organisms that are normally not disease-producing may cause life-threatening infections in these people.
Parasitic Infection
Protozoa and other parasites are rare causes of lung diseases in the United States, but an infection known as paragonimiasis, caused by a flatworm known as the lung fluke, is prevalent in tropical regions worldwide. The protozoan Pneumocystis carinii causes a form of pneumonia that has become a common illness in adults who have AIDS.