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close this bookTuberculosis Control in Refugee Situations - An Inter-Agency Field Manual (World Health Organisation, 1997, 72 p.)
close this folder1. TUBERCULOSIS (TB)
View the document1.1 Global Burden of TB
View the document1.2 Natural History of TB
View the document1.3 TB in Refugee Situations
View the document1.4 HIV/TB

1.2 Natural History of TB

It is estimated that up to one-third of the world's population is infected with the TB organism. Once infected, a person stays infected for many years, probably for life. The vast majority (90%) of people who are infected with the TB organism do not develop active TB disease. In these healthy, asymptomatic, but infected individuals, the only evidence of infection may be a positive tuberculin skin test.

Transmission occurs by airborne spread of infectious droplets. The source of infection is a person with TB of the lung who is coughing.

Infected persons can develop active TB disease at any time. The chance of developing TB disease is greatest shortly after infection and then steadily lessens as time goes by. Various physical or emotional stresses may trigger the progression of infection to disease. The most important trigger is weakening of immune resistance, especially by HIV infection. TB can affect most tissues and organs, but most commonly the lungs.

Without treatment, after 5 years, 50% of active pulmonary TB patients will be dead, 25% will be healthy (self-cured by a strong immune defence), and 25% will remain ill with chronic, infectious TB.