In 1901, while examining pathologic specimens of a spleen from a patient who had died of kala azar he observed oval bodies and published his account of them in 1903. Charles Donovan of the Indian Medical Service independently found such bodies in other kala azar patients, and they are now known as Leishman-Donovan bodies, and recognized as the protozoan which causes kala azar, Leishmania donovani. Synonyms for kala azar now include leishmaniasis.
Leishman also helped elucidate the life cycle of Spirochaeta duttoni, which causes African tick fever, and, with Almroth Wright, helped develop an effective anti-typhoid inoculation.