Screening of latent infection by Mycobacterium tuberculosis (LTBI) and treatment of positive subjects is the key point in the prevention of TB; such a strategy should be performed mainly among individuals with risk of progression. Progression from LTBI to active TB is highest both in recent contacts of patients with active TB, and in immunocompromised subjects. Therefore, LTBI could either be considered a permanent host defence or, seen from the opposite point of view, it could represent a long-lasting threat if the efficiency of the immune system declines over a lifetime. Interferon gamma release assays (IGRAs) show better sensitivity and specificity than the Mantoux test.