Pathogenesis: The parasites cause three distinct types of clinical disease, cutaneous, mucocutaneous and visceral leishmaniasis. Old World cutaneous leishmaniasis is caused by L. tropica and L. aethiopica while New World cutaneous leishmaniasis is caused by L. mexicana and L. braziliensis. Infections generally involve only one or a few lesions at the bite site; they do not spread to other sites. Active lesions appear as open sores/ulcers with pronounced inflammation. Most lesions heal spontaneously, leaving the host with solid protective immunity to re-infection. However, under certain conditions (esp. immuno-compromised hosts), some L. aethiopica infections may spread giving rise to disseminated cutaneous leishmaniasis (not unlike leprosy in appearance). Infections by L. braziliensis are also often confined to single skin lesions, but sometimes they spread to the mucocutaneous junction in the pharynx and may cause severe destructive nasopharyngeal lesions. Visceral leishmaniasis is caused by L. donovani whereby infected macrophages congregate in the viscera, notably the liver and spleen, producing hepatosplenomegaly, oedema and anaemia. It is a slow but progressive illness, with bouts of irregularly recurring fever, and is invariably fatal, unless treated.