What Is It?
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.