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U.S. Department of Health and Human Services


Core Curriculum on Tuberculosis, 2000

Chapter 2
Transmission and Pathogenesis


TB is spread from person to person through the air. When a person with pulmonary or laryngeal TB coughs, sneezes, speaks, or sings, droplet nuclei containing M. tuberculosis are expelled into the air. Depending on the environment, these tiny particles (1-5 microns in diameter) can remain suspended in the air for several hours.

If another person inhales air containing droplet nuclei, transmission may occur. The probability that TB will be transmitted depends on four factors:

  1. the infectiousness of the person with TB (the number of organisms expelled into the air),
  2. the environment in which exposure occurred,
  3. the duration of exposure, and
  4. the virulence of the organism. (See Infection Control in Health Care Facilities for more information on infectiousness.)
The best way to stop transmission is to isolate patients with infectious TB immediately and start effective TB therapy. Infectiousness declines rapidly after adequate therapy is started, as long as the patient adheres to the prescribed regimen.

Persons at the highest risk of becoming infected with M. tuberculosis are close contacts — persons who had prolonged, frequent, or intense contact with a person with infectious TB. Close contacts may be family members, roommates, friends, coworkers, or others. Data collected by CDC since 1987 show that infection rates have been relatively stable, ranging from 21% to 23% for the contacts of infectious TB patients2.

Among contacts of persons with drug-resistant TB, infection rates seem to be similar. However, because they may have a poor response to treatment, persons with drug-resistant disease are often infectious for longer periods and therefore have the potential to infect more contacts. HIV-positive persons with TB disease are not considered more infectious than HIV-negative persons with TB disease.

Extrapulmonary TB is rarely contagious (except for laryngeal TB); however, transmission from extrapulmonary sites has been reported during aerosol-producing procedures, such as autopsies and tissue irrigation3,4,5.  


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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