Tuberculous lymph node. Light micrograph of a tuberculous lymph node 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 here), as well as to other body organs. Here, the lymph node has been almost destroyed by necrosis (tissue death) as the tubercle (site of TB infection) has enlarged.