Tuberculosis is a chronic disease caused by the bacteria Mycobacterium
tuberculosis. The initial disease may not be noticed, but eventually the infection may
spread to various parts of the body (for example: tuberculosis of the bones and joints,
intestinal tuberculosis, and urinary tract tuberculosis). The incidence of tuberculosis is
less today than in the past. In the United States Army in World War II, the number of
tuberculosis cases was 1 per 1,000 soldiers. By 1982, only 250,000 cases were
reported in the United States. The decline in the number of cases can be attributed to
three factors: education about causes of tuberculosis; better diet and nutritional
information; and early diagnosis and treatment of tuberculosis cases.
COMMUNICABILITY OF INFECTION
a. Reservoir, Sources, Transmission Route. The most common reservoir of
tuberculosis is man. The source of infection is respiratory secretion of individuals who
have active pulmonary lesions. Tuberculous bacteria may be transmitted by direct or
indirect contact with the patient who has open lesions. The most common route of
transmission, however, is by inhaling airborne droplet nuclei. When a patient coughs or
sneezes, a person may inhale the bacteria in the air and, if susceptible, may contract
tuberculosis. Family, military, and institutional living can contribute to the spread of
tuberculosis if a person has an active case that is unrecognized and, consequently,
untreated. A reservoir for Mycobacterium bovis, the bovine strain of tuberculosis, is
diseased cattle. Man ingests this bacillus by drinking the raw milk from such cattle.
Man may also contract this type of tuberculosis by inhaling airborne organisms in and
around barns that have infected animals or by handling infectious animal products.
Since herds of animals are continuously tested for tuberculosis and milk is pasteurized,
this form of tuberculosis has been controlled.
b. Incubation Period. The incubation period is usually about four to six weeks
from the time of effective exposure to the appearance of a primary lesion of first
infection. The time between the first infection and reinfection of tuberculosis may be
many years. If the individual is reinfected, the length of time necessary for new lesions
to develop depends on the general health of the person and the source and numbers of
organisms entering his system.