Etiology and Epidemiology

Many pathogens have been associated with infectious pneumonia. Bacterial varieties include Escherichia coli, Hemophilus influenzae, Klebsiella pneumoniae, Legionella pneumophila, Mycobacterium tuberculosis, staphylococci, and streptococci. Common viral agents are arbovirus, cytomegalovirus, influenza, measles, respiratory syncytial virus, and varicella. Other pathogens include Mycoplasma pneumoniae, Blastomyces, Nocardia, Pneumocystis carinii, and the rickettsial pathogen Coxiella burnetii. This list is far from exhaustive.

Despite the large number of pneumonia pathogens, the disease develops only if other host or environmental conditions are met. Normally the airways and lung tissue distal to the throat (glottis) are sterile. Occasionally, organisms that are always present in the upper airway, in the digestive tract, or on the skin enter the lung. Ordinarily they are rapidly eliminated either by mechanical means, such as by coughing and the microscopic action of cilia, or by immune mechanisms. Infection and the resultant inflammation of pneumonia can occur in healthy individuals, but are often associated with a breakdown in one or more of the usual defense mechanisms or, more rarely, with exposure to a particularly virulent strain of pathogen or an unusually high aerosol dose of organism (as in Legionnaire's disease). Occasionally, bacterial pneumonia will occur as a result of septicemic spread from an infectious focus elsewhere in the body.

Immune defenses are altered by underlying debility, be it nutritional (starvation and alcoholism), infectious (tuberculosis and AIDS), neoplastic (cancer or lymphoma), or iatrogenic. Iatrogenic causes of immune depression are becoming more important with the increasingly frequent use of immunosuppressive or cytotoxic drugs in the treatment of cancer, autoimmunity, and organ transplantation. One special form of immune deficiency resulting from absent splenic function leads to an exaggerated susceptibility to S. pneumoniae infection and lobar pneumonia. This condition, called functional asplenia, can arise following splenectomy or as a complication of sickle-cell anemia. Thus a relative predisposition to pneumococcal infection can be found in the geographic regions containing a high frequency of hemoglobin S.

Mechanical defenses are hampered by immobility due to altered consciousness, paralysis or pain, endotracheal intubation, and prior viral infection of the upper airway, such as bronchitis or a cold. Controversy surrounds the ancient etiologic theory about cold temperatures. August Hirsch (1886) found a high incidence of pneumonia in months having wide variability in temperature. Two factors do tend to support an indirect correlation between cold and pneumonia: Predisposing viral infections are more common in winter, and some evidence suggests that the mechanical action of cilia is slowed on prolonged exposure to cold.

Lobar pneumonia appears in all populations. Its incidence and mortality rate are higher in individuals or groups predisposed to one or more of the factors described above. Elderly patients frequently develop pneumonia as the terminal complication of other debilitating illness, hence the famous metaphor "friend of the aged" (Osier 1901).

Mortality rates for pneumonia are difficult to estimate because of its multifactorial nature and the fact that it can complicate other diseases. As mentioned previously, untreated lobar pneumonia can result in death in 30 percent of cases. With antibiotics, fatalities are reduced to a varying extent depending on the underlying condition of the patient, but in persons over the age of 12 years the mortality is at least 18 percent and in immunocompromised persons it is much higher. In the late nineteenth century, Hirsch suggested that the annual death rate averaged 1.5 per 1,000 in a survey of European and American municipal statistics (Hirsch 1886). Pollution of the atmosphere may have contributed to the apparent rise in pneumonia mortality in Britain during the last half of the nineteenth century (Howe 1972). William Osier saw pneumonia mortality as one of the most important problems of his era and applied to it John Bunyan's metaphor (originally intended for tuberculosis), "Captain of all these men of death." Contemporary pneumonia mortality combined with influenza is still the sixth most common cause of death in the United States, where mortality is estimated to be approximately 0.3 per 1,000 (U.S. Public Health Service Centers for Disease Control 1989). Pneumococcal vaccination of high-risk groups with functional asplenia and other forms of debility has contributed to a decrease in incidence.

Other forms of pneumonia follow epidemiological patterns that reflect the frequency of the organism or causative toxins in the host's environment, as for example Legionella, which favors water in air-conditioning systems.

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