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Lung Disease & Respiratory Health Center

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Tuberculosis (TB) - What Happens

Tuberculosis (TB) develops when Mycobacterium tuberculosis bacteria are inhaled into the lungs. The infection usually stays in the lungs. But the bacteria can travel through the bloodstream to other parts of the body (extrapulmonary TB).

An initial (primary) infection can be so mild that you don't even know you have an infection. In a person who has a healthy immune system, the body usually fights the infection by walling off (encapsulating) the bacteria into tiny capsules called tubercles. The bacteria remain alive but cannot spread to surrounding tissues or other people. This stage is called latent TB, and most people never go beyond it.

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A reaction to a tuberculin skin test is how most people find out they have latent TB. It takes about 48 hours after the test for a reaction to develop, which is usually a red bump where the needle went into the skin. Or you could have a rapid blood test that provides results in about 24 hours.

If a person's immune system becomes unable to prevent the bacteria from growing, the TB becomes active. Of people who have latent TB, 5% to 10% (1 to 2 people out of 20) will develop active TB at some point in their lives.1

Active TB in the lungs

Active TB in the lungs (pulmonary TB) is contagious. TB spreads when a person who has active disease exhales air that contains TB-causing bacteria and another person inhales the bacteria from the air. These bacteria can remain floating in the air for several hours. Coughing, sneezing, laughing, or singing releases more bacteria than breathing.

In general, after 2 weeks of treatment with antibiotics, you cannot spread an active pulmonary TB infection to other people.

Skipping doses of medicine can delay a cure and cause a relapse. In these cases, you may need to start treatment over. Relapses usually occur within 6 to 12 months after treatment. Not taking the full course of treatment also allows antibiotic-resistant strains of the bacteria to develop, making treatment more difficult.

Without treatment, active TB can cause serious complications, such as:

  • Pockets or cavities that form in the lungs. These damaged areas may cause bleeding in the lungs or may become infected with other bacteria and form pockets of pus (abscesses).
  • A hole that forms between nearby airways in the lungs.
  • Difficulty breathing because of blocked airways.

TB can be fatal if it is not treated.

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