The species commonly found in East Africa and the Indian subcontinent is L. donovani and the species found in Europe, North Africa, and Latin America is L. infantum, also known as L. chagasi.The insect vectors are species of sandfly of the genus Phlebotomus in the Old World, and of Lutzomyia in the New World. Sandflies are tiny flies, measuring 3–6 mm long by 1.5–3 mm in diameter, and are found in tropical or temperate regions throughout the world. The sandfly species Lutzomyia longipalpis is the primary vector of this disease. The larvae grow in warm, moist organic matter (such as old trees, house walls, or waste) making them hard to eradicate.Visceral Leishmaniasis/kala-azar samples from India revealed the presence of not only the primary causative protozoan parasite, i.e. Leishmania donovani (LD) but also co-infection with another protozoan member called Leptomonas seymouri (LS). The latter parasite (LS) further contained a RNA virus known as Leptomonas seymouri narna-like virus 1 (Lepsey NLV1). So, it appears that a great majority of kala-azar patients in  the Indian subcontinent are exposed to a RNA virus in LS, the co-infecting parasite with LD i.e. the "LD-LS-Lepsey NLV1 triple pathogen" phenomenon. Life cycle
The life cycle of Leishmania is completed in two hosts, humans and sandflies. The adult female sandfly is a bloodsucker, usually feeding at night on sleeping prey. When the fly bites an individual infected with Leishmania, the pathogen is ingested along with the preys blood.
In the words of Jill Seaman, the doctor who led relief efforts in the Upper Nile for the French organization Médecins Sans Frontières, "Where else in the world could 50% of a population die without anyone knowing?" Due to the South Sudanese Civil War, kala-azar has spread rapidly among the population. The Indian medical practitioner Upendra Nath Brahmachari was nominated for the Nobel Prize in Physiology or Medicine in 1929 for his discovery of ureastibamine (an antimonial compound for the treatment of kala-azar) and a new disease, post kala-azar dermal leishmaniasis. Brahmacharis cure for visceral leishmaniasis was the urea salt of para-amino-phenyl stibnic acid which he called Urea Stibamine.During the nineteenth century, kala-azar was discovered near moving bodies of water in southeast Asia. Dr. Jean Dow and Dr. William McClure, are credited with finding the cure for the disease in China. Largely uncredited for her contribution, Dr. Jean Dow was one of the first to isolate the microorganism in China and conduct clinical studies on its origin. This work continued under Ernest Struthers and Lionel Napier at the School of Tropical Medicine at Calcutta to discover that kala-azar was transmitted by sandflies. Research
Combination drug therapies are currently under investigation, particularly by the Drugs for Neglected Diseases initiative (DNDi). Combination therapies allow for the use of existing drugs in combination, each in lower doses, which helps to decrease the incidence of severe side effects and drug toxicity, as well as the risk for development of resistance against the drugs; they have been shown to be cost-effective strategies.