Leishmaniasis is a disease caused by protozoan parasites of the genus Leishmania. The parasite migrates to the internal organs such as the liver, spleen (hence "visceral"), and bone marrow, and, if left untreated, will almost always result in the death of the host.Kala-azar: A chronic and potentially fatal parasitic disease of the viscera (the internal organs, particularly the liver, spleen, bone marrow and lymph nodes) due to infection by the parasite called Leishmania donovani. ... The term "kala-azar" comes from India where it is the Hindi for black fever.