any of several species of flagellate protists belonging to the genus Leishmania in the order Kinetoplastida. These protists are parasites of vertebrates, to which they are transmitted by species of Phlebotomus, a genus of bloodsucking sand flies. The leishmanial parasites assume two forms: a round or oval leishmanial stage, which lives and multiplies in the vertebrate host; and an elongate, motile, flagellated organism called a leptomonad, which is found in the alimentary tract of the sand fly. In their leishmanial stage, the organisms are taken in with the meal of the fly, and they develop into leptomonads in the fly's stomach and multiply there. They eventually migrate to the fly's mouthparts, from which the leptomonads enter the wound made at the next feeding, thus initiating a new infection.

There are three separate species in the genus Leishmania: these three species look quite alike but cause three different human diseases that are collectively called leishmaniasis. L. donovani, which attacks the liver, spleen, bone marrow, and other viscera, causes kala-azar (q.v.) in Africa, Europe, and Asia. L. tropica causes oriental sore (q.v.) in Africa, Europe, and the East; lesions that range from pimples to large ulcers are formed on the skin of the hands, feet, legs, and face. L. brasiliensis, the cause of American leishmaniasis in Central and South America, produces similar skin lesions but also causes deeper lesions of the oral and nasal mucous membranes.