For More Information US National Library of Medicine and National Institutes of Health Centers for Disease Control and Prevention US Department of Labor Occupational Safety and Health Administration American Lung Association

Tuberculosis, or TB, is an infectious disease that primarily affects the lungs. As a matter of fact, about 85% of all TB cases affect the lungs.

Mycobacterium [mI-kO-bak-'tir-E-um] tuberculosis is the bacterium that causes TB. TB is spread from person to person through the air. When a person with infectious TB disease (TB that can be spread) coughs or sneezes, tiny particles containing Mycobacterium tuberculosis can be expelled into the air. These droplets containing the tuberculosis bacterium can remain suspended in the air for several hours, depending on the environment. These airborne particles may then be inhaled into other people's lungs as they breathe, leading to infection and colonization of the Mycobacterium tuberculosis.

TB infection means that tubercle bacilli are in the body but the body’s immune system is keeping the bacilli under control. The immune system does this by producing special immune cells called macrophages that surround the tubercle bacilli. The cells form a hard shell that keeps the bacilli contained and under control. Once in the lungs the bacteria may remain undetected and remain dormant (latent) for years or may progress to active tuberculosis at any time.

Created: September 2003
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