Definition: Tuberculosis is a disease that affects the lungs, and it can also affect the mouth, nose, throat, esophagus, and other parts of the body. Tuberculosis occurs when bacteria enter the body through the skin or through the air that comes in contact with the skin during an infection. The body's immune system attacks the cells in the lungs and other organs to fight off the bacteria, but it can also spread to other parts of the body if the infection is not treated properly.