It is assumed that some features like intra-macrophage habitat of the Leishmania parasites, rare availability of infected macrophages in peripheral blood for vector sandflies to suck in, short flight range of sandflies, non-availability of an animal reservoir encountered in visceral leishmaniasis indicate slow and limited transmission potential and even so epidemics occur every 15 to 20 yr. To verify if these assumptions are true, the natural history of kala-azar was studied, using an endemic village in Bihar (India) as an unit of study, over a period of 5 yr (1984-1988). Village Jethuli is bound by the river Ganga on the north and separated from neighbouring endemic villages on other three sides by agricultural land, is isolated entomologically (as regards sandflies). The village has a population of 3236 persons of different social status and depending on economic conditions have three types of dwellings, brick made with cement plaster, brick made with mud plaster and mud houses. The first case of kala-azar was reported in a migrant from district Vaishali on the other side of Ganga where kala-azar appeared in an epidemic form. Studies showed that the infection is built up slowly, first in the same house and then in the immediate neighbourhood. In this village, maximum number of cases occurred in 1984 and 1985, and they were treated by our Institute and cured. In subsequent years, only a few cases occurred (i.e., 6 in 1986 and 4 in 1987) while no case occurred in 1988.(ABSTRACT TRUNCATED AT 250 WORDS)