Natural course of disease

Most exposure to tuberculosis occurs in childhood as a result of breathing in M. tuberculosis transmitted from infected, usually adult, individuals. Once inhaled, the bacteria multiply at the site of infection in the lung and produce an inflamed area. They may then move to the lymph glands in the lungs, which can enlarge. This is known as primary tuberculosis.

In a minority of individuals the bacteria can spread throughout the body leading to serious disease. In most cases, however, the body's immune defences contain the M. tuberculosis organisms by walling them off in scar nodules within the lungs. Here, some of the infection can persist and remain latent for years or even decades. Some people with latent tuberculosis will go on to develop active disease during their lifetime. This is known as reactivation tuberculosis. Most reactivation disease develops during the first few years after infection, but sometimes it occurs decades later.