Even though tuberculosis (TB) can be caused by three different bacteria, the term almost always refers to the contagious, potentially fatal infection that is caused by the bacterium Mycobacterium tuberculosis. In the United States, TB usually is acquired by inhaling air that contains bacteria coughed into the air by a person with active TB. The bacteria can remain in the air for several hours.
During the course of most TB infections, the bacteria are destroyed. In fact, almost all TB infections heal without ever being noticed; there's only a slight chance of developing an active infection within 1 to 2 years. However, the bacteria can remain dormant for years. About 80% of TB infections occur after dormant bacteria become active -- for example, when the immune system is weakened, in which case the infection can be life threatening. Active TB usually begins in the lungs, and, if spread through the blood, can damage other parts of the body.