Leishmania donovani is a protozoan parasite that causes visceral leishmaniasis, a severe disease affecting internal organs, particularly the liver and spleen. It is transmitted to humans through the bite of infected female sandflies, primarily in tropical and subtropical regions. The infection can lead to symptoms such as fever, weight loss, and anemia. If left untreated, visceral leishmaniasis can be fatal. Diagnosis typically involves blood tests or bone marrow examination, and treatment options include medications like antimonials and liposomal amphotericin B.