A bacterial
infection caused by
Mycobacterium tuberculosis.
TB bacteria are spread by airborne droplets expelled from the lungs when a person with active
TB coughs, sneezes or speaks. Repeated exposure to these droplets can lead to
infection in the air sacs of the lungs. The immune defenses of healthy people usually prevent
TB infection from spreading beyond a very small area of the lungs. If the body's
immune system is impaired because of
infection with HIV, aging, malnutrition or other factors, the
TB bacterium may begin to spread more widely in the lungs or to other tissues.