Tuberculosis is caused by infection with the bacterium mycobacterium tuberculosis, which spreads in microscopic droplets that are released into the air when someone with untreated, active TB coughs, sneezes, speaks, or spits. However, despite the fact that the TB bacterium enters the air almost any time someone with the active disease opens their mouth, the disease is not that easy to catch. By contrast, the flu spreads much more easily. Usually, it requires long-term contact with a person who has an active TB infection to become infected yourself.

Someone with latent TB cannot transmit the disease to anyone else. A person with active TB who has undergone treatment for at least two weeks is generally considered no longer contagious.

When a person becomes infected with TB, it primarily attacks the lungs. The immune system mounts a response between two and eight weeks after the infection and sometimes it can eradicate the bacterium completely. More often, the immune system successfully sequesters the tuberculosis bacteria in the lungs like a scab sealing a wound. When this happens, the infection is called latent and may persist this way for years.

If the immune system fails to wall off the bacteria initially, or if the bacteria are able to escape from their cage due to deficiencies in the immune system, the bacteria can multiply and fill the air spaces of the lungs. At this point it is called an active TB infection and it tends to spread throughout the lungs and sometimes to other parts of the body. A person with an active TB infection can spread it to others.