latent TB infection
Latent TB infection is the body's stalemate with the bacterium. The person has been infected, but their immune system has walled the bacteria off and kept them in check, so there are no symptoms, the chest looks normal, and the infection cannot be passed to anyone else. It is like a fire that has been smothered down to glowing coals — not burning, but not fully out.
Inside small granulomas, dormant or very slowly replicating bacteria persist for years or decades. The person feels well and is not contagious, but they carry a lifetime risk that the truce could break down. Roughly 5–10% of healthy infected people will progress to active disease at some point, with the highest risk in the first two years after exposure.
Latent infection is detected indirectly, by testing the immune system's memory of TB rather than finding the bacteria themselves — typically with a tuberculin skin test or an interferon-gamma release assay. Treating latent infection with one or two drugs, when appropriate, greatly lowers the chance of later active disease, which is why screening high-risk groups matters for control.
A healthcare worker with no symptoms has a positive interferon-gamma release assay and a normal chest X-ray; she is diagnosed with latent TB and offered preventive treatment.
Infection present, disease absent — the definition of latent TB.
Risk of reactivation rises sharply with anything that weakens immunity — HIV, certain cancers, organ transplant medication and TNF-blocking drugs are important examples.