A disease caused by the protozoan Leishmania, which is usually transmitted by the bites of infected sand flies. There are four major types of leishmaniasis: cutaneous, diffuse, mucocutaneous, and visceral. Visceral leishmaniasis affects internal organs and is characterized by fever, weight loss, enlargement of the spleen and liver, and a drop in blood cell production with possible anemia. In people with AIDS, visceral leishmaniasis is the most common type, and, if left untreated, it is nearly always fatal.