This Symposium honours the achievements of Dr Karel Styblo. In this presentation, specific epidemiologic insights are reviewed. Studies of the epidemiology of tuberculosis in Eskimos showed a picture of tuberculosis at the height of the epidemic. Very high incidence was observed in young people who experienced a high fatality rate. Application of specific control measures were accompanied by rapid decline in rates, greater than observed in any other human population, demonstrating that tuberculosis could be brought under control by specific intervention. Studies of the natural trend of tuberculosis in South India showed that, even in the absence of intervention, a decline was observed in the rates of this disease. In the absence of chemotherapy, 50 per cent of cases die within 5 years, 30 per cent recover spontaneously and 20 per cent remain sputum positive. Studies of the efficacy of BCG in Madras, enabled to study the impact of efficient case-finding associated with poor treatment results showing that such a situation multiplies the number of surviving, infectious cases in the community and, thus, actually deteriorates the epidemiological situation. These various basic studies have shown both how to create success and how to create failure in tuberculosis programmes.