Causes of Tuberculosis.

Mycobacterium tuberculosis, the bacterium that causes tuberculosis,spreads in microscopic droplets that are released into the air when someone with the untreated, active form of the disease coughs, speaks, laughs, sings or sneezes.
Although tuberculosis is contagious, it's not especially easy to catch. In general, you need long-term contact with an infected person to become infected yourself. You're much more likely to contract tuberculosis from a family member or close co-worker than from a stranger on a bus or in a restaurant. A person with nonresistant active TB who's been effectively treated for at least two weeks is generally no longer contagious. Rarely, a pregnant woman with an active TB disease may pass the bacteria to her fetus. 

TB infection versus active TB
Although TB can affect other organs and tissues, it primarily attacks your lungs. Approximately two to eight weeks after your lungs are infected with M. tuberculosis, your immune system springs into action. Macrophages — specialized white blood cells that ingest harmful organisms — begin to surround and "wall off" the tuberculosis bacteria in your lungs, much like a scab forming over a wound. If the macrophages are successful, 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 the TB skin test, but you won't feel sick or have symptoms and you can't transmit the disease to others.