Parasitic protozoa belonging to the genus Leishmania are the cause of a spectrum of diseases in humans including chronic long-term infections.

Depending on the infecting species and host immunity, these parasites may induce localized, self-curing, cutaneous lesions (cutaneous leishmaniasis) or disseminate to facial mucosal tissues (mucocutaneous) or the liver and spleen (visceral) to cause very disfigurative and life-threatening mucocutaneous and visceral forms of leishmaniasis.

Parasites can persist long term in the host, even after apparent resolution of the disease, a factor that may be important in the development of long-term immunity to subsequent infection, but also the cause of re-occuring infections in immunocompromised individuals.

Depending on the Leishmania species, amastigotes can reside within individual tight-fitting phagolysosomes or within large communal phagolysosomes. Propagation of intracellular amastigotes to other macrophages occurs following cell division or lysis of the host cell. Leishmania are stealthy pathogens that either avoid or actively suppress macrophage microbicidal processes.

Infected sandfly takes a bloodmeal on a human or another animal host -> injection of pathogenic metacyclic promastigotes into the subdermal layers of the skin -> Promastigotes can be phagocytosed by a number of host cell types (neutrophils, dendritic cells and fibroblasts) -> propagation ocurrs primarily within macrophages -> Internalized promastigotes are delivered to the mature phagolysosome compartment of host macrophages where they differentiate to the non-motile amastigote stage.

There are some 500 known phlebotomine species, but only about 30 have been found to transmit leishmaniasis. Only the female sandfly transmits the parasites. Female sandflies need blood for their eggs to develop, and become infected with the Leishmania parasites when they suck blood from an infected person or animal. Over a period of between 4 and 25 days, the parasites develop in the sandfly.

Leishmania species that have reached vertebrates and mammals as hosts, including humans, are varied phylogenetically and biologically. Together with the differential responses of the host's immune system, those factors present a wide range of species infecting humans and the type of clinical presentations.

Due to the early presence of these protist organisms in evolution and their interaction of invertebrates, before the separation of the current continents, there are different Leishmania species and disease spectrum across continents.

Cutaneous leishmaniasis is the most common form of the disease. It usually produces ulcers on the exposed parts of the body, such as the face, arms and legs. There may be a large number of lesions – sometimes up to 200 – which can cause serious disability. When the ulcers heal, they invariably leave permanent scars.

Mucocutaneous leishmaniasis, the lesions can lead to partial or total destruction of the mucous membranes of the nose, mouth and throat cavities and surrounding tissues. This disabling form of leishmaniasis is highly incapacitating and disfigurative.

Visceral leishmaniasis, also known as kala-azar, is characterized by irregular bouts of fever, substantial weight loss, swelling of the spleen and liver, and anaemia.