Kala Azar, also known as visceral leishmaniasis, is a severe parasitic infection caused by Leishmania donovani and transmitted by sandfly bites. It primarily affects internal organs like the spleen, liver, and bone marrow, leading to prolonged fever, weight loss, anemia, and an enlarged spleen and liver. Without timely treatment, Kala Azar can be fatal, so diagnosis typically involves blood tests, serology, and spleen or bone marrow aspirates to detect the parasite.