Tuberculosis bacteria, illustration. Tuberculosis (TB) is an infectious disease caused by the bacterium Mycobacterium tuberculosis. TB is spread by the inhalation of droplets coughed or sneezed by an infected person. The bacteria multiply and form a localized tuberculosis bacterial colony (tubercle) in the lungs. Untreated infections can spread to other parts of the body, and can be fatal. Symptoms of tuberculosis include blood-containing sputum, fever, night sweats and weight loss.