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?
A rapid and dramatic increase in the occurrence of a disease
is referred to as an epidemic.
Studies of migrant populations can be used to help distinguish
whether a disease is more environmentally or genetically determined.
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
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)