Macrophages in tuberculosis. Illustration of macrophages (pink) engulfing tuberculosis bacteria (bacilli, yellow) in a pulmonary alveolus in the lungs. Macrophages engulf and digest cellular debris and pathogens. Tuberculosis (TB), an infectious disease caused by the bacterium Mycobacterium tuberculosis, 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 include blood-containing sputum, fever, night sweats and weight loss.