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Education Materials > Publications > Self-Study Modules on TB > Module 1 > Summary

Self-Study Modules on Tuberculosis

This is an archived document. The links are no longer being updated.

Module 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 skin test.

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 sites.

Additional Reading
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; 1992.

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.


Released October 2008
Centers for Disease Control and Prevention
National Center for HIV/AIDS, Viral Hepatitis, STD, and TB Prevention
Division of Tuberculosis Elimination -

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