Tuberculosis is an infectious bacterial disease caused by Mycobacterium tuberculosis and is known to spread through the air. When infected persons cough, spit, or talk, the M. tuberculosis organisms are dispersed into the air. Only a small number of M. tuberculosis is enough to cause an infection when inhaled. However, not all people infected with M. tuberculosis will become sick; the immune system either kills or walls off the germs where they can lie dormant for years. Failure of the immune system to control infection with M. tuberculosis leads to active disease. Tuberculosis and acquired immunodeficiency syndrome (AIDS) form a lethal combination. AIDS weakens the immune system, and a person that is human immunodeficiency virus (HIV)-positive and infected with M. tuberculosis is much more likely to become sick with tuberculosis than someone who is HIV-negative and infected with M. tuberculosis.