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Tuberculosis (TB) Cause


Mycobacterium tuberculosis bacteria, the cause of TB.
Credit: NIAID

Tuberculosis is caused by the infectious agent known as Mycobacterium tuberculosis (Mtb). This rod-shaped bacterium, also called Koch's bacillus, was discovered by Dr. Robert Koch in 1882.


  • Mtb is a small, slow-growing bacterium that can live only in people. It is not found in other animals, insects, soil, or other nonliving things.
  • Mtb is an aerobic bacterium, meaning it needs oxygen to survive. For this reason, during active TB disease, Mtb complexes are always found in the upper air sacs of the lungs.

TB Infection

When a person breathes in Mtb-contaminated air, the inhaled TB bacteria reach the lungs. This causes an Mtb infection. However, not everyone infected with TB bacteria becomes sick. The bacteria can remain dormant (asleep) for years and not cause any TB disease. This is called latent TB infection. People who have latent TB infection do not get sick and do not spread the bacteria to others. But, some people with latent TB infection eventually do get TB disease.

So it is important to get the appropriate treatment and get rid of the bacteria even in latent TB infection.

TB Disease

For someone to develop active TB disease, the following two events must take place:

  1. The bacteria enter the body and cause an Mtb infection.
  2. The immune system cannot stop the TB bacteria from growing and spreading after the initial infection.

The active bacteria multiply and destroy the tissue. A person with TB disease shows symptoms that vary, depending on where the TB bacteria are growing. In most cases, the bacteria attack the lungs.

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Last Updated March 05, 2012