TB bacteria become active if the immune system cannot stop them from growing. The active bacteria begin to multiply in the body and cause TB disease. Some people develop TB disease soon after becoming infected, before their immune system can fight the TB bacteria. Other people may get sick later, when their immune system becomes weak for some reason. Babies and young children often have a weak immune system. People infected with HIV, the virus that causes AIDS, have a very weak immune system.

From there, they move through the blood to other parts of the body, such as the kidney, spine, and brain. TB in the lungs or throat can be infectious. This means that the bacteria can spread to other people. TB in other parts of the body, such as the kidney or spine, is usually not infectious.

People with TB disease are likely to spread TB to people who come in close contact and spend time with them everyday. This includes family members, friends, and coworkers.