Reactivation or reinfection in adult tuberculosis: Is that the question?

Looking at the chapter on "natural history" in any tuberculosis (TB) reference book, there is a kind of certainty regarding TB in adults. That is the concept of "post-primary" TB described as the reactivation of dormant bacilli hidden in an old lesion developed during infancy due to a type of local immunosuppression. Intriguingly, this concept involves at least two major uncertainties: how can dormant bacilli remain for such a long period, almost a lifetime, in an old lesion, taking into account granuloma dynamism; and what sort of local immunosuppression is the one that facilitates reactivation? The controversy between reactivation and exogenous reinfection as the cause of active TB started very soon in TB research. Interestingly, this "balance" was disturbed in the 1960s when the "Unitary Concept" became very successful in supporting the reactivation dogma. The "Unitary Concept" was mainly based on the data of tuberculin surveillance during the pre-antibiotic era as well as the data obtained from experimental modelling in animals. At the same time, the "Three-risks model" appeared to explain the relationship between the risk of infection and TB incidence, granting reinfection a key role in adult TB together with primary infection. This role was reinforced by the studies of recurrence based on molecular epidemiology, and a better knowledge of the immune response, granuloma dynamics, and lung physiology. Now it is a matter of taking it into account when designing new prophylactic and therapeutic strategies and also reflecting it in text books to better illustrate to our students.