TB is caused by an organism called Mycobacterium tuberculosis that is spread from person to person through the air. M. tuberculosis organisms are sometimes called tubercle bacilli. When a person with infectious TB disease coughs or sneezes, droplet nuclei containing tubercle bacilli may be expelled into the air. Other people may inhale the air containing these droplet nuclei and become infected.
TB infection begins when the tubercle bacilli multiply in the small air sacs of the lungs. A small number enter the bloodstream and spread throughout the body, but the body's immune system usually keeps the bacilli under control. People who have latent TB infection (LTBI) but not TB disease do not have symptoms of TB, and they cannot spread TB to others. They usually have a positive reaction to the tuberculin skin test or QuantiFERON TB Gold Test ®.
In some people who have LTBI, the immune system cannot keep the tubercle bacilli under control and the bacilli begin to multiply rapidly, causing TB disease. This can happen very soon after TB infection or many years after infection. About 10% of people who have LTBI will develop disease at some point, but the risk is greatest in the first year or two after infection, than for other people.
TB disease usually occurs in the lungs (pulmonary TB), but it can also occur in other places in the body (extrapulmonary TB). Miliary TB occurs when tubercle bacilli enter the bloodstream and are carried to all parts of the body, where they grow and cause disease in multiple sites.