The leishmaniases include a complex of vector-borne diseases, caused by more than 20 species of the protozoan genus Leishmania, and ranging from localized skin ulcers to lethal systemic disease. Leishmaniasis is classified as one of the "most neglected diseases," based on the limited resources invested in diagnosis, treatment and control, and its strong association with poverty.

The leishmaniases are transmitted to humans in sylvatic, domestic and peridomestic cycles ranging from cities to deserts and rain forests on every continent except Australia and Antarctica. In the Western Hemisphere, leishmaniasis is found in some parts of Mexico, Central America, and South America. It is not found in Chile or Uruguay. In the Eastern Hemisphere, leishmaniasis is found in parts of Asia, the Middle East, Africa, and southern Europe. There is no transmission in Australia or the Pacific islands.

The majority of human cases occur in a small number of countries:

• 90% of visceral leishmaniasis cases occur in parts of India, Bangladesh, Nepal, Sudan, Ethiopia and Brazil
• 90% of cutaneous leishmaniasis cases occur in parts of Afghanistan, Algeria, Iran, Saudi Arabia, Syria, Brazil, Colombia, Peru and Bolivia 

Climate change and other environmental changes have the potential to expand the geographic range of the vectors and leishmaniasis transmission in the future.

Dozens of different sand fly vectors, adapted to the different ecological settings where leishmaniasis occurs, are known to transmit some form of leishmaniasis.  Leishmaniasis, especially the visceral form, can also be transmitted by blood transfusion or sharing of contaminated needles.  Congenital transmission has been reported, but appears to be rare.

In sylvatic cycles, such as those in New World rain forests and the deserts of Central Asia, animal reservoir hosts can maintain transmission indefinitely without human disease. Sporadic or epidemic leishmaniasis occurs when humans enter the sylvatic habitat for economic or military purposes, or when human habitation encroaches on the sylvatic setting. In domestic cycles, humans or dogs form the predominant or sole infection reservoir. In the Mediterranean basin and parts of Latin America, visceral leishmaniasis transmission is zoonotic (dog – sand fly – human). The area that account for the largest number of human cases, for example, visceral leishmaniasis in South Asia and cutaneous leishmaniasis in Afghanistan, usually reflect anthroponotic (human - sand fly - human) transmission.