PNEUMONIA

inflammation and consolidation of the lung tissue as a result of infection, inhalation of foreign particles, or irradiation. Many organisms, including viruses and fungi, can cause pneumonia, but the most common causes are bacteria, in particular species of Streptococcus and Mycoplasma. Although viral pneumonia does occur, viruses more commonly play a part in weakening the lung, thus inviting secondary pneumonia caused by bacteria. Fungal pneumonia can develop very rapidly and may be fatal, but it usually occurs in hospitalized persons who, because of impaired immunity, have reduced resistance to infection. Contaminated dusts, when inhaled by previously healthy individuals, can sometimes cause fungal lung diseases. Mycoplasmal pneumonia, caused by Mycoplasma pneumoniae, an extremely small organism, usually affects children and young adults; few cases beyond the age of 50 are seen. Most outbreaks of this disease are confined to families, small neighbourhoods, or institutions, although epidemics can occur. M. pneumoniae grows on the mucous membrane that lines the surfaces of internal lung structures; it does not invade the deeper tissuesmuscle fibres, elastic fibres, or nerves. The bacteria can produce an oxidizing agent that might be responsible for some cell damage. Usually the organism does not invade the membrane that surrounds the lungs, but it does sometimes inflame the bronchi and alveoli. Symptoms do not appear until about three weeks after the initial infection. Headaches and a run-down feeling increase to feverishness, muscle pain, and sore throat. As the disease progresses, coughing becomes the major symptom. Sputum discharge may contain flecks of blood. Any chest pains result from the tenderness of the trachea (windpipe) and muscles from severe coughing. The illness may be severe, but there are few fatalities. Recovery generally occurs in a few weeks with the help of antibiotic drugs. Streptococcal pneumonia, caused by Streptococcus pneumoniae, is the single most common form of pneumonia, especially in hospitalized patients. The bacteria may live in the bodies of healthy persons and cause disease only after resistance has been lowered by other illness or infection. Viral infections such as the common cold promote streptococcal pneumonia by causing excessive secretion of fluids in the respiratory tract. These fluids provide an environment in which the bacteria flourish. This is generally a more severe illness than mycoplasmal pneumonia, although most patients recover with antibiotic treatment. Another bacterium, Klebsiella pneumoniae, although it has little ability to infect the lungs of healthy persons, produces a highly lethal pneumonia that occurs almost exclusively in hospitalized patients with impaired immunity. Other bacterial pneumonias include Legionnaires' disease (q.v.), caused by Legionella pneumophilia; pneumonia secondary to other illnesses caused by Staphylococcus aureus and Hemophilus influenzae; and psittacosis (q.v.), an atypical infectious form. One of the major causes of death among AIDS patients has been Pneumocystis carinii pneumonia. Pneumonia can also result from inhalation of oil droplets, which scar the lung surfaces. This type of disease occurs most frequently in workers exposed to large quantities of oily mist and in the elderly. Oil that is being swallowed may be breathed into the respiratory tract, or, less often, it may come from the body itself when the lung is physically injured. Scar tissue forms as a result of the presence of the oil. Ordinarily no treatment is necessary. Inflammation of lung tissues may result from X-ray treatment of structures within the chest. The disease makes its appearance from 1 to 16 weeks after exposure to the X rays has ceased. Recovery is usual unless too great an area of lung tissue is involved.