According to a 2011 (news) article in Science
More than a third of the world's population is estimated to be infected with Mycobacterium tuberculosis. Most people's immune system can keep the bacteria in check, but there is a lifetime chance of 1 in 10 that the dormant infection will progress to TB; the disease still kills 4000 people every day.
N.B. according to Wikipedia (citing the WHO)
People with latent TB do not spread the disease.
Also, Wikipedia mentions that latent TB infections are diagnosed through tuberculin skin test. But looking at the article Wikipedia links for the latter, it mentions quite a few caveats for this test:
Tuberculin skin testing
This test measures a patient's immune response to M. tuberculosis antigens (tuberculin). A small amount of tuberculin is injected intradermally and the skin reaction is measured two or three days later.
The test is very sensitive for detecting tuberculosis in healthy individuals if 5 mm induration is used to define a positive reaction. However, many conditions result in false negative reactions, including active tuberculosis. Bacillus Calmette-Gurin (BCG) vaccination and exposure to environmental nontuberculous mycobacteriosis cause intermediate size reactions. Sensitivity is often sacrificed by choosing larger indurations to define a positive reaction based on the incidence of tuberculosis and the extent of non-specific cross-reactivity in the population being tested. Box 2 provides general recommendations for categorising skin reactions, but regional tuberculosis control units should be consulted for local guidelines.
Box 2: Criteria for defining a tuberculin skin testing reaction as positive *
≥5 mm – in people with recent exposure (within 2 years) to tuberculosis + high risk for progression to active disease (e.g. <5 years of age, HIV infection, other immunosuppressive illness; see Table 1)
≥10 mm – in people with recent exposure to tuberculosis, regardless of BCG vaccination status; all non-BCG vaccinated people except for those with both low lifetime risk for tuberculosis infection and residence in geographical areas where exposure to environmental nontuberculous mycobacteriosis is common
≥15 mm – in all people regardless of BCG vaccination status
* This refers to the induration produced by an intradermal injection of purified protein derivative (PPD) equivalent to 5 units of PPD-S. These criteria are meant as suggestions only. Local tuberculosis control units should be consulted for local guidelines.
So it seems rather non-trivial to assess the tuberculin skin test results, i.e. there are quite a few confounding factors and the criteria for judging a test positive [at least in that paper] are influenced by other factors linked to the potential risk of developing active disease (e.g. HIV infection).
So, given all those complications, going back to the first quote (from Science), how well is the percentage of latent TB infections bracketed/known? I.e. what's the confidence interval for true latent TB infections?
