Tuberculosis is caused by a bacterium known as Mycobacterium tuberculosis. (The related bacteria Mycobacterium bovis and Mycobacterium africanum can also cause tuberculosis.) The body's response to active TB infection produces inflammation that can damage the lungs. Areas affected by active TB gradually fill with scar tissue.
Tuberculosis (TB) is a contagious or infectious disease. It is spread from person-to-person. A person is often infected by inhaling the germs. These germs have been sprayed into the air by someone with the active disease who coughs.
However, inhaling the germ does not mean you will develop active disease. A person's natural body defenses are often able to control the infection so that it does not cause disease. In this case, the person would be infected, but does not have active disease. Only about 10% of those infected will develop TB in their lifetimes.
Active disease can occur in an infected person when the body's resistance is low or if there is a large or prolonged exposure to the germs that overcome the body's natural defenses. The body's response to active TB infection produces inflammation that can damage the lungs. The amount of damage may be quite extensive even though the symptoms may be minimal.