Self-Study Modules on Tuberculosis
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1: Transmission and Pathogenesis
TB has affected humans for centuries. Until the 1940s and 1950s, there
was no cure for TB. Many people with TB were sent to sanatoriums,
special rest homes where they followed a prescribed routine every
day. After drugs were discovered to treat TB, many people with TB
were cured, and the death rate for TB dropped dramatically.
TB is caused by an organism called Mycobacterium tuberculosis
that is spread from person to person through the air. M. tuberculosis
organisms are sometimes called tubercle bacilli. When a person with
infectious TB disease coughs or sneezes, droplet nuclei containing
tubercle bacilli may be expelled into the air. Other people may
inhale the air containing these droplet nuclei and become infected.
TB infection begins when the tubercle bacilli multiply in the small
air sacs of the lungs. A small number enter the bloodstream and
spread throughout the body, but the body's immune system usually
keeps the bacilli under control. People who have TB infection but
not TB disease do not have symptoms of TB, and they cannot spread
TB to others. They usually have a positive reaction to the tuberculin
In some people who have TB infection, the immune system cannot
keep the tubercle bacilli under control and the bacilli begin to
multiply rapidly, causing TB disease. This can happen very soon
after infection or many years after infection. About 10% of people
who have TB infection will develop disease at some point, but the
risk is greatest in the first year or two after infection. Also,
the risk is higher for people with certain medical conditions, such
as HIV infection, than for other people.
TB disease usually occurs in the lungs (pulmonary TB), but it can
also occur in other places in the body (extrapulmonary TB). Miliary
TB occurs when tubercle bacilli enter the bloodstream and are carried
to all parts of the body, where they grow and cause disease in multiple
American Thoracic Society. Diagnostic standards and classification
of tuberculosis. Am Rev Respir Dis. 1990;142:725-735.
Bates B. Bargaining for Life: A Social History of Tuberculosis,
1876-1938. Philadelphia: University of Pennsylvania Press;
Core Curriculum on Tuberculosis, 3rd ed. Atlanta: Centers
for Disease Control and Prevention; 1994.
DuBos R, DuBos J. The White Plague: Tuberculosis, Man, and
Society. New Brunswick, NJ: Rutgers University Press; 1987.
Ryan F. The Forgotten Plague: How the Battle Against Tuberculosis
Was Won — And Lost. Boston: Little, Brown and Co; 1992.