Tuberculosis (TB) is a contagious bacterial infection that most frequently affects the lungs. In addition, TB involves other areas of the body, such as the lymph nodes, genitourinary tract, bones, joints, the membranes covering the brain (meninges), and the membranes covering the digestive organs (peritoneum).
Transmission of TB usually occurs as a result of prolonged contact with an infected person. Most people who are exposed to TB do not develop an active infection. Older people and those with HIV or cancer are more likely to develop an active infection. People with a history of TB exposure have about a 10% lifetime risk of developing the active disease. People with HIV and a history of TB exposure have about a 10% yearly risk of developing the active disease.
Tuberculosis infection can be either active or latent. Tuberculosis can exist as a latent infection, usually in the lungs. If you have a latent infection, you will not feel ill, and in most cases will not be contagious to others. If your immune system becomes compromised, the latent infection may become active, and you will feel ill and become contagious.
Tuberculosis is caused by airborne bacteria spread most commonly from person to person by inhaling contaminated droplets of coughs and sneezes. Mycobacterium tuberculosis is the bacterium that most commonly causes TB. Air becomes contaminated when a person with active tuberculosis coughs out the bacteria, which may then remain in the air for several hours. Once inhlaled by another person, the infection will usually gravitate towards the base of the lungs. There, the bacteria multiply slowly and spread to nearby lymph nodes (a sort of drainage system for the body). If the bacteria spread from the lymph nodes to the blood, the infection may then travel to other organs in the body.
Other closely related organisms, such as Mycobacterium bovis (transmitted in unpasturized milk in developing countries) and Mycobacterium africanum, can cause the disease; however, they are less common, especially in the United States. Casual, one-time exposure to TB bacteria rarely causes an infection. Rather, one must experience prolonged exposure to an individual with an active infection (for example, living with an infected family member, or working in close proximity to someone with an active infection) in order for transmission to occur.
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