Causes of tuberculosis

Tuberculosis is caused by an organism called Mycobacterium tuberculosis. The bacteria spread from person to person through microscopic droplets released into the air. This can happen when someone with the untreated, active form of tuberculosis coughs, speaks, sneezes, spits, laughs or sings. Rarely, a pregnant woman with active TB may pass the bacteria to her unborn child.

Although tuberculosis is contagious, it's not especially easy to catch. You're much more likely to get tuberculosis from a family member or close co-worker than from a stranger. Most people with active TB who've had appropriate drug treatment for at least two weeks are no longer contagious.

TB infection vs. active TB
If you breathe TB bacteria into your lungs, one of four things might happen:

• You don't become infected with TB. Your immune system immediately destroys the germs and clears them from your body.
• You develop latent TB infection. The germs settle in your lungs and begin to multiply. Within several weeks, however, your immune system successfully "walls off" the bacteria in your lungs, much like a scab forming over a wound. The bacteria may remain within these walls for years — alive, but in a dormant state. In this case, you're considered to have TB infection and you'll test positive on a TB skin test. But you won't have symptoms and won't transmit the disease to others.

• You develop active TB. If your immune defenses fail, TB bacteria begin to exploit your immune system cells for their own survival. The bacteria move into the airways in your lungs, causing large air spaces (cavities) to form. Filled with oxygen — which the bacteria need to survive — the air spaces make an ideal breeding ground for the bacteria. The bacteria may then spread from the cavities to the rest of your lungs as well as to other parts of your body.

  If you have active TB, you're likely to feel sick. Even if you don't feel sick, you can still infect others. Without treatment, many people with active TB die. Those who survive may develop long-term symptoms, such as chest pain and a cough with bloody sputum, or they may recover and go into remission.

• You develop active TB years after the initial infection. After you've had latent TB for years, the walled-off bacteria may suddenly begin multiplying again, causing active TB, also known as reactivation TB. It's not always clear what triggers this reactivation, but it most commonly happens after your immune system becomes weakened. Your resistance may be lower because of aging, drug or alcohol abuse, malnutrition, chemotherapy, prolonged use of prescription medications such as corticosteroids or TNF inhibitors, and diseases such as HIV/AIDS.

Only about one in 10 people who have TB infection goes on to develop active TB. The risk is greatest in the first two years after infection and is much higher if you have HIV infection.