Progression of tuberculosis. Cutaway computer illustration showing Mycobacterium tuberculosis bacteria (yellow) - the causative agent of tuberculosis - entering a person's airways and travelling to the lungs (lower left and right). Once in the lungs the bacteria infect macrophage white blood cells. This then causes macrophages, T lymphocytes, B lymphocytes, and fibroblasts to aggregate and form granulomas (yellow lumps, lower left), with lymphocytes surrounding the infected macrophages. The granulomas then cause abnormal cell death (necrosis), with the affected tissue replaced by cavities of necrotic (dead) tissue (black).