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Tuberculosis

Condition Overview:

Tuberculosis is a contagious infection caused by an airborne bacterium, Mycobacterium tuberculosis.

Tuberculosis usually affects the lungs, although it can attack almost any organ in the body. Other mycobacteria (such asMycobacterium bovis orMycobacterium africanum) occasionally can cause a similar disease.With most infectious diseases (such as strep throat or pneumonia), a person becomes sick right after the microorganism enters the body and is noticeably ill within 1 or 2 weeks. Tuberculosis does not follow this pattern.Stages of Infection Except for very young children, few people become sick immediately after tuberculosis bacteria enter their body (primary infection). Many tuberculosis bacteria that enter the lungs are immediately killed by the body's defenses. Those that survive are captured inside white blood cells called macrophages. The captured bacteria can remain alive inside these cells in a dormant state for many years, walled off inside tiny scars (latent infection). In 90 to 95% of cases, the bacteria never cause any further problem, but in about 5 to 10% of infected people they start to multiply (active disease). It is in this active phase that an infected person actually becomes sick and can spread the disease.

More than half the time, activation of dormant bacteria happens within the first 2 years, but it may not occur for a very long time. Doctors do not always know why the dormant bacteria become active, but it often occurs when the person's immune system becomes impaired—for example, from very advanced age, the use of corticosteroids, or AIDS. Like many infectious diseases, tuberculosis spreads more quickly and is much more dangerous in people who have a weakened immune system. For such people (including the very young, the very old, and those who are also infected with HIV), tuberculosis can be life threatening.

Transmission of Infection Mycobacterium tuberculosis can live only in people; it cannot be carried by animals, insects, soil, or other nonliving objects. A person can be infected with tuberculosis only from another person who has active disease. Touching someone who has the disease does not spread it, because the bacteria are transmitted only through the air. Mycobacterium bovis, a bacterium that can live in animals, is an exception. In developing countries, children become infected with it by drinking unpasteurized milk from infected cattle.

People with active tuberculosis in their lungs contaminate the air with bacteria when they cough, sneeze, or even speak. These bacteria can stay in the air for several hours. If another person breathes them in, that person may become infected. People who have latent disease or tuberculosis that is not in their lungs do not spread bacteria into the air and cannot transmit the infection.