Pulmonary tuberculosis. Light micrograph of tissue from a lung in a case of pulmonary tuberculosis. Tuberculosis (TB) is a disease caused by infection with Mycobacterium tuberculosis bacteria. The bacteria are usually inhaled in water droplets, and can progress from the lungs to the lymphatic system, as well as to other body organs. There is an area of necrosis (tissue death) at left, surrounded by macrophages.