Microepidemics of tuberculosis: the stone-in-the-pond principle.

In a low prevalence country with no systematic BCG vaccination and a low frequency of environmental mycobacterial infections, for each recognized case of tuberculosis or infection with M. tuberculosis source detection should be undertaken. Sometimes this search will lead to discovery of a microepidemic. The extent of contact screening in a microepidemic can easily be determined if observed group prevalences are compared to expected group prevalences. The described method illustrates this search for contacts in concentric circles around the source case: the stone-in-the-pond principle.