Pulmonary tuberculosis. Illustration of a human lung diseased with pulmonary tuberculosis. At centre right is the affected lung seen sectioned through; at left is the heart with its associated blood vessels. In pulmonary tuberculosis the air sac alveoli of the lung become filled with an inflammatory exudate (yellow). The exudate is made of macrophages (white blood cells) and bacteria which cause the infection Mycobacterium tubercu losis. The walls of alveoli have disintegrated in places, forming cavities (upper centre). Lesions or tubercles of infection are visible (white). Tuberculosis is an infectious lung disease which, although still a killer, can now be cured.