Tuberculosis (TB) develops when Mycobacterium tuberculosis bacteria are inhaled into the lungs. The infection usually stays in the lungs, but the bacteria can travel through the bloodstream to other parts of the body (extrapulmonary TB).

An initial (primary) infection can be so mild that you don't even know you have an infection. In a person who has a healthy immune system, the body usually fights the infection by walling off (encapsulating) the bacteria into tiny capsules called tubercles. The bacteria remain alive but cannot spread to surrounding tissues or other people. This stage is called latent TB, and most people never go beyond it.

A reaction to a tuberculin skin test is how most people find out they have latent TB. It takes about 48 hours after the test for a reaction to develop, which is usually a red bump where the needle went into the skin. Or you could have a blood test (QuantiFERON-TB Gold) that provides results in about 24 hours.

If a person's immune system becomes unable to prevent the bacteria from growing, the TB becomes active. Of people who have latent TB, 5% (1 person in 20) will develop active TB within 2 years after the initial infection. Another 5% of people who have latent TB will develop active TB at some point in their life.¹

People who have latent TB may be at risk for developing the active disease if they:

• Have a condition or disease that weakens their immune system, such as human immunodeficiency virus (HIV) infection, some cancers, or poorly controlled diabetes.
• Have poor access to health care, such as homeless people, migrant farm workers, or people who abuse alcohol or drugs.
• Take medications that contain corticosteroids for a long time or are taking tumor necrosis factor-alpha antagonists (used to treat rheumatoid arthritis or Crohn's disease).
• Have a condition that results in an impaired immune system, which can occur in older adults, newborns, women who have recently given birth, or people who have had an organ transplant and are taking medications to prevent organ rejection.
• Have a chronic lung disease caused by breathing in tiny sand or silica particles (silicosis).
• Have celiac disease.²
• Have had gastric bypass surgery or a gastrectomy.
• Are 10% or more under their healthy body weight.

Active TB in the lungs

Active TB in the lungs (pulmonary TB) is contagious. TB spreads when a person who has active disease exhales air that contains TB-causing bacteria and another person inhales the bacteria from the air. These bacteria can remain floating in the air for several hours. Coughing, sneezing, laughing, or singing releases more bacteria than breathing. TB is more likely to spread in situations where:

• People are living together in crowded conditions. TB can spread rapidly in nursing homes, hospitals, homeless shelters, schools, military barracks, and prisons.
• People live in the same house with a person who has active TB. This increases the chance of inhaling TB-causing bacteria and developing an infection. TB is not spread by handling objects that have been touched by a person who has TB.