There is a growing body of evidence supporting the role of humans as a source of Old World anthroponotic Leishmania. Circumstantial evidence is consistent with the participation of humans as reservoirs in the transmission of New World leishmaniasis. Leishmaniasis has been considered to affect persons working or living mainly in forested areas, where the transmission cycle involved wild animals and sylvan sandflies. However, consequent to deforestation patterns of human migration and urbanisation, both parasites and vectors have proved capable of adapting to the domestic cycle, where other mammals (e.g. dogs, horses, opossums) and even humans may act as alternative reservoirs. If so, indeed this is the case, it would explain the sudden appearance of outbreaks in areas where leishmaniasis has not been previously reported, as well as the perpetuation of transmission in established foci.