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.
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