TB can cause illness in many ways, sometimes at the time of infection, but often years later.

Many people are exposed to TB as children, when they breathe in TB germs that have been expelled into the air from the lungs of a person infected with TB.  However people can become infected at any point in their lives.

The TB germs multiply in the lungs, cause inflammation, and move to the lymph glands – the command centres of the immune system - in the lungs. This phase of the illness is called primary TB. The TB can spread and grow and cause disease. In most HIV-negative people, the immune system contains the TB by forming a wall of scar tissue around it and most people, about 80%, clear the infection. In the remaining 20% of people, the TB will remain alive, but dormant. Although a person may not feel ill because of TB, the TB germ can remain alive within this contained area for many years, even decades, causing illness at a later time. This type of TB is usually called latent TB.

TB that is causing illness is called active TB.

TB that resurfaces from scar tissue and causes illness after a period of time is called reactivation TB.

Unlike most other HIV-related opportunistic infections (so called because they take the opportunity of the body’s weakened immune defences to develop), TB can occur in people with normal CD4 cell counts and can be transmitted to other people, HIV-positive and HIV-negative.