Tuberculosis (TB) is a disease caused by bacteria called Mycobacterium tuberculosis. TB usually affects the lungs, but it can also affect other parts of the body. TB is spread through the air from one person to another. The bacteria are put into the air when a person with active TB disease of the lungs or throat coughs or sneezes. People nearby may breathe in these bacteria and become infected. If not treated properly, TB disease can be fatal.