TB Infection Is Not TB Disease

Pathogenesis

When a person inhales air that contains droplets, most of the larger droplets become lodged in the upper respiratory tract (the nose and throat), where infection is unlikely to develop. However, the droplet nuclei may reach the small air sacs of the lung (the alveoli), where infection begins. The following section describes the pathogenesis of TB (the way TB infection and disease develop in the body).

At first, the tubercle bacilli multiply in the alveoli and a small number enter the bloodstream and spread throughout the body. Bacilli may reach any part of the body, including areas where TB disease is more likely to develop. These areas include the upper portions of the lungs, as well as the kidneys, the brain, and bone. Within 2 to 10 weeks, however, the body's immune system usually intervenes, halting multiplication and preventing further spread. The immune system is the system of cells and tissues in the body that protect the body from foreign substances.

TB Infection

TB infection means that tubercle bacilli are in the body but the body's immune system is keeping the bacilli under control. The immune system does this by producing special immune cells that surround the tubercle bacilli. The cells form a hard shell that keeps the bacilli contained and under control.

TB infection is detected by the tuberculin skin test. Most people with TB infection have a positive reaction to the tuberculin skin test.

People who have TB infection but not TB disease are NOT infectious — in other words, they cannot spread the infection to other people. These people usually have a normal chest x-ray. It is important to remember that TB infection is not considered a case of TB. Major similarities and differences between TB infection and TB disease are shown in Table 1.1.

TB Disease

Some people with TB infection develop TB disease. TB disease develops when the immune system cannot keep the tubercle bacilli under control and the bacilli begin to multiply rapidly. The risk that TB disease will develop is higher for some people than for others.

TB disease can develop very soon after infection or many years after infection. In the United States, about 5% of the people who have recently been infected with M. tuberculosis will develop TB disease in the first year or two after infection. Another 5% will develop disease later in their lives. In other words, about 10% of all people who have TB infection will develop disease at some point. The remaining 90% will stay infected, but free of disease, for the rest of their lives.

Progression of TB

People who are exposed to TB may or may not develop TB infection. People with TB infection may or may not develop TB disease. The risk of developing TB disease is highest in the first 2 years after infection.

Because about half the risk of developing TB disease is concentrated in the first 2 years after infection, it is important to detect new infection early. People with TB infection can be given treatment to prevent them from getting TB disease. Thus, detecting new infection early helps prevent new cases of TB.