The important differences in the infectivity of the various forms of tuberculosis can be explained by quantitative data concerning the behaviour of the tubercle bacillus in man and the number of bacilli in the lesions and sputum. Patients in whom tubercle bacilli can be detected by direct examination of the sputum smear are the main sources of transmission. Moreover the individuals infected by them break down more often with the disease. In the individual patient, the use of antibacterial drugs completely changes the natural history of the disease: not only do patients no longer die but they are cured; their period of infectivity is considerably reduced, relapses are avoided, chronicity disappears. The drugs used prophylactically in individuals of high risk groups prevent development of the disease. The impact of chemotherapy is reflected by a two-to-three-fold increase in the speed of decline of the risk of infection, a decline which had started before the introduction of the drugs. While patients given the right combination of drugs lose their infectivity in a few weeks (probably most often in less than two weeks), treatment must of course be continued much longer and regularly in order to ensure the maintenance of conversion and the absence of relapse. This stresses the importance of providing means to ensure the taking of the drugs by all patients. The future reduction of transmission will essentially depend on the maintenance of an adequate system ensuring the early diagnosis and correct treatment of cases, which will inevitably continue to appear among the already infected portion of the population. Epidemiological surveillance is mandatory as well as the surveillance of the delivery of services, particularly of the quality of diagnosis and therapeutic services. The roles of public health authorities and perhaps still more that of the practising physician, specialized and not specialized, remain considerable both from an epidemiological point of view and from the point of view of the relief of all the suffering still created by the disease.