Tuberculosis

What Is It?

Tuberculosis (TB) is a bacterial infection that kills more than 2 million people a year. Most of these deaths occur in developing countries. The bacterium that usually causes TB in humans is Mycobacterium tuberculosis.

About one-third of the world's population is infected with TB. However, most do not show signs of the disease. In these people, the bacteria are inactive (latent) and cannot be transmitted to others. If the body's immune system weakens, however, TB can become active and cause disease. About 10 percent of people with the inactive form of the disease eventually will develop active tuberculosis.

TB was once a leading cause of death in the United States, but after the discovery in the 1940s of drugs to treat it, the number of TB cases declined. Today in the United States, health experts estimate that 10 million to 15 million people are infected with inactive TB. Many of these people were born in other countries and exposed to TB before coming to the United States. Other groups at higher risk of infection include the poor, the homeless and people who have been in prison or abuse substances.

Worldwide, TB is second only to HIV in causes of death by infectious disease among adults. In addition, many developing countries are suffering dual epidemics of TB and HIV. Because people with HIV have weakened immune systems, those with inactive TB infection are at high risk of developing active tuberculosis. In fact, TB is a major cause of death for people infected with HIV.

TB typically affects the lungs. But in up to one-third of infected people, the illness also involves other areas of the body. Common sites of infection include the lymph nodes, the membranes that cover the brain (meninges), the joints, the kidneys and the membrane covering the digestive organs (peritoneum).

TB bacteria are spread from person to person through the air. The bacteria are in droplets of secretions that come out of your mouth or nose when you cough or sneeze. One-time exposure to someone with TB is not likely to cause infection. Repeated or prolonged exposure is usually necessary. Touching someone with TB or sharing his or her utensils will not lead to infection, because the bacteria infect the lungs only when they are inhaled into the lungs

When infection occurs, a bacteria-filled droplet is inhaled into the deepest portion of the lung, where the bacteria reproduce (replicate) and spread through the body. At this point, the immune system generally can keep the bacteria from replicating any more, but usually cannot destroy them completely. The disease usually remains in this inactive or dormant state for life. People with inactive TB have no symptoms, and there is no way to tell they have been infected except for a special skin test.

In about 10 percent of people with inactive TB, the bacteria eventually manage to start replicating again, causing active TB. People whose immune systems are weakened by HIV are much more likely to develop active disease. Each year 5 percent to 10 percent of these people develop active TB.