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    The distribution of a disease within a population can be characterized by three basic questions: Who develops the disease? Where does the disease occur? When does the disease occur?
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    A rapid and dramatic increase in the occurrence of a disease is referred to as an epidemic.
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    Studies of migrant populations can be used to help distinguish whether a disease is more environmentally or genetically determined.

image A 34-year-old female domestic worker who had recently emigrated from Southeast Asia to the United States came to the emergency room with a 6-week history of cough, fever, night sweats, weakness, fatigue, and shortness of breath. Previously she had been in good health. She had experienced two uncomplicated pregnancies and deliveries, followed by a tubal sterilization. Cavitary lesions were visible on the patient’s chest x-ray. A smear of a sputum specimen revealed acid-fast bacilli. Mycobacterium tuberculosis subsequently grew from cultures of the sputum, and these organisms were susceptible to all drugs tested. The patient was placed on an initial antibiotic regimen involving four drugs administered daily under direct observation by the health care provider. After 2 weeks of daily therapy, the patient improved clinically and she was maintained on directly observed, four-drug therapy daily for the next 6 weeks. The patient remained asymptomatic, and there was no evidence of bacilli in her sputum. Her treatment regimen was reduced to two drugs administered three times each week, and she remained under direct observation for an additional 18 weeks.

The patient resided with her husband and two young children in an apartment building. Tuberculin skin tests were administered to each of the family members at the time of the patient’s initial diagnosis, and results were positive for the patient’s husband and 3-year-old daughter. Although no evidence of clinically active tuberculosis was found in either the spouse or daughter, preventive therapy was administered to all three family members. Skin testing of 54 other residents of the apartment building revealed one other infected adult, who lacked evidence of active disease and received preventive antibiotic therapy. None of the tuberculin skin tests administered to the patient’s contacts at work were positive.

Tuberculosis is caused by mycobacteria transmitted on small airborne particles that are created when an individual with pulmonary tuberculosis coughs or sneezes. Air currents circulate these particles throughout an entire room or building. When a susceptible person inhales these particles, tubercle bacilli may become established in the lungs and spread throughout the body. Usually the host’s immune system contains this initial infection within a short period of time. A small proportion (10%) of patients will develop active clinical illness months or years later, when the mycobacteria begin to replicate and cause symptoms.

As shown in Table 3–1, environmental as well as personal factors affect the likelihood of tuberculosis transmission. Each of the environmental features listed tends to increase the concentration of mycobacteria in the air. Transmission also is promoted by (1) characteristics ...

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