Tuberculosis 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 tuberculosis in humans is Mycobacterium tuberculosis.
About one-third of the world's population is infected with tuberculosis. 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, tuberculosis can become active and cause disease.
Worldwide, tuberculosis is second only to the human immunodeficiency virus (HIV) in causes of death by infectious disease among adults. Many developing countries are suffering dual epidemics of tuberculosis and HIV. The interaction between these two diseases has been labeled "Toxic synergy" because each epidemic impacts people in the same impoverished regions of the world, and because each worsens the other. People with HIV have weakened immune systems, so they are more likely to acquire a new case of tuberculosis, or to develop reactivation of latent disease. Those with tuberculosis are more likely to die if they are co-infected with HIV.
Tuberculosis typically affects the lungs. But in up to one-third of infected people, particularly those with HIV/AIDS, 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).
Tuberculosis 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 tuberculosis is not likely to cause infection. Repeated or prolonged exposure is usually necessary. Touching someone with tuberculosis 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 usually 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 tuberculosis do not have any symptoms, and there is no way to tell they have been infected except for a special skin test.
Active tuberculosis occurs in several different forms:
Primary pulmonary tuberculosis – In about 5% of people, the immune system can't stop the initial tuberculosis infection. These people develop active tuberculosis within one year of exposure to the bacteria. This type of active tuberculosis is most common in infants and children, especially in developing countries with high rates of malnutrition and poor medical care. People with HIV and other diseases that suppress the immune system are also at risk.
Postprimary (reactivation) pulmonary tuberculosis – About 95% of people infected with tuberculosis can inactivate the disease at first. Most of them never develop active disease. In those that do, the bacteria eventually overcome the immune system and begin to replicate and spread, usually in the lungs. The bacteria may destroy large areas of the lungs, forming cavities filled with bacteria and dead cells.
Extra-pulmonary tuberculosis – Tuberculosis also can become active in other parts of the body, whether or not the lungs are involved. Common sites of infection include the bones, kidneys, lymph nodes and central nervous system.
Disseminated or miliary tuberculosis – Tuberculosis can spread through the entire body by way of the bloodstream.