example, during a procedure to irrigate a tuberculosis-infected abscess, during laboratory processing of infected tissue, or during an autopsy or embalming.5

Whether produced by coughing or other actions, most of the largest, heaviest bacterium-bearing particles will settle fairly quickly and harmlessly on surfaces such as walls, furniture, and clothing. Some airborne particles will reach another person’s nose and throat but will be trapped and then removed when the person swallows or spits. The smallest, lightest particles (called droplet nuclei) may reach the lung’s alveoli, which are the tiny, endmost parts of the respiratory tract and the starting point for most tuberculosis infections. In poorly ventilated spaces, droplet nuclei may stay in the air for hours or even days. The bacteria are very sensitive to sunshine and other ultraviolet light.

Extended, close, indoor contact is usually required for tuberculosis to be transmitted from one person to another. At least one report has, however, documented transmission after workplace exposure limited to a few minutes (Templeton et al., 1995).

Tuberculosis is considered moderately infectious, with infection developing in perhaps 30 to 50 percent of those who have extended, close, indoor contact with people with active disease. Measles, by contrast, is considered highly infectious, with infection developing in an estimated 80 percent of susceptible people who come in contact with an infected individual (Haas and Des Prez, 1995).

The likelihood that M. tuberculosis will be transmitted from one person to another depends on several factors. These factors include

  • the infectiousness of a person with infectious tuberculosis, which is related to the number of bacteria he or she expels and, possibly, the virulence (disease-causing potential) of the bacteria;

  • the length of exposure to an infectious person or to air contaminated with tuberculosis bacteria;

  • the environment surrounding an infectious person, for example, the size of a room and how well it is ventilated; and

  • the functioning of an exposed person’s immune system.

Infection with M. tuberculosis

Typically, when tuberculosis bacteria reach the alveoli in the lungs, they attract and are engulfed by white blood cells called macrophages.6 The bacteria multiply within the macrophages, and some may escape and


Transmission by other routes has also been reported (e.g., from inadequately sterilized bronchoscopes, accidental self-inoculation, and a kidney transplant) (Dooley and Tapper, 1997).


This is a greatly simplified description of a very complex pathological process, aspects of which are still not fully understood.

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