INFECTION, REINFECTION OR REACTIVATION - THE MOLECULAR EPIDEMIOLOGIC EVIDENCE

The recent availability of effective molecular tools for the character ization of strain diversity and relatedness among Mycobacterium tuberc ulosis isolates allows re-evaluation of many concepts on transmission, infection, re-infection, re-activation and relapse: (i) the proportio n of cases of tuberculosis due to recent transmission is higher than p reviously suspected, and most contact investigations fail to identify transmission routes; (ii) reinfection after prior active disease is no t rare, in particular among HIV-infected individuals; (iii) false-posi tive cultures for M. tuberculosis occur frequently in some settings; ( iv) the identification of 'families' of related strains and of prevale nt clones in some populations raises important issues about selection pressures and virulence.