The tuberculin skin test is used to identify persons who have been infected with M. tuberculosis. Most infected people have a positive reaction to the tuberculin skin test within 2 to 10 weeks after infection. Persons who are infected with M. tuberculosis but who do not have TB disease are not infectious to others. TB infection in a person who does not have TB disease is not considered a case of TB and is often referred to as latent TB infection. TB infection progresses to disease when tubercle bacilli overcome the defenses of the immune system and begin to multiply.
Infection can progress to disease very quickly or many years after infection.
Some medical conditions increase the risk that TB infection will progress to disease. Compared with immunocompetent persons, who are infected with M. tuberculosis, infected persons who are immunosuppressed are at considerably greater risk of developing TB disease. In an HIV-infected person, TB disease can develop in either of two ways. First, a person who has TB infection can become infected with HIV, and then TB disease can develop as the immune system is weakened. Second, a person who has HIV infection can become infected with M. tuberculosis, and TB disease can then rapidly develop.