Tuberculosis. Light micrograph of a human lung affected with tuberculosis. The air sac alveoli of the lung can be seen, filled with an inflammatory exudate. This exudate consists largely of macro-phages (white blood cells) and the infecting bacteria Mycobacterium tuberculosis. At lower right are numerous blood vessels (red); the walls of alveoli have disintegrated (at lower left). Tuberculosis is an infectious lung disease which once was a major killer disease worldwide. In the lungs, bacteria breathed in form inflammatory lesions known as “tubercles” which destroy lung tissue. Magnification: 320 at 35mm size / ASTRID & HANNS-FRIEDER MICHLER/SCIENCE PHOTO LIBRARY