TB is an airborne disease caused by the bacterium Mycobacterium tuberculosis (M. tuberculosis) (Figure ). M. tuberculosis and seven very closely related mycobacterial species (M. bovis, M. africanum, M. microti, M. caprae, M. pinnipedii, M. canetti and M. mungi) together comprise what is known as the M. tuberculosis complex. Most, but not all, of these species have been found to cause disease in humans. In the United States, the majority of TB cases are caused by M. tuberculosis. M. tuberculosis organisms are also called tubercle bacilli
