The parasite’s life cycle is quite simple. When an infected sandfly bites, the parasite (as a promastigote) is injected directly into the skin. This unicellular parasite then penetrates the cells of the reticuloendothelial system (macrophages), where it multiplies in the form of amastigotes (the non-flagellate form) ("a" = without; "mastix" = whip). It is this form that can be seen in a skin biopsy or bone marrow aspirate. Multiplication results in bursting of the host cell, whereupon other cells become infected.

When another sandfly later bites, these infected cells can be ingested. The parasite is then still located in infected macrophages. The blood meal in the stomach is completely surrounded by a peritrophic membrane. The parasite transforms into a different form (promastigote with flagellum) in the insect and then multiplies. After 2-3 days the peritrophic membrane is digested and the parasites are released into the lumen of the stomach and intestine. They then attach to the microvilli of the intestine by means of their flagellae. They produce a chitinase which damages the chitin coating of oesophageal-gastric junction, so that the valve between stomach and oesophagus no longer functions adequately and leaks, resulting in a backflow of parasites to the mouthparts. The parasites accumulate 7 to 10 days later in the insect’s proboscis and can be injected when the insect bites its next victim. The insect is infectious 7-10 days after an infected meal and has to survive for this time in order to be transmitted. Haemoglobin degradation products inhibit the secretion of chitinase and/or inhibit the enzyme itself making backflow of parasites to the mouthparts more difficult. Certain plant sugars do not have this effect. The insects also feed on plant juices. A balance between plant and animal feeding is required for successful transmission. A botanical description of the vector’s environment (biotope) can be important in scientific studies.

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The intestine of the sandfly consists of three major sections: an anterior intestine (cibarium, pharynx, oesophagus or gullet and oesophageal crop or gizzard), a middle intestine (stomach; the cranial part is called the cardia) and a posterior or terminal intestine (ileum, rectum). The transition from anterior to middle intestine is formed by a small valve (stomodeum valve). The transition from middle intestine to terminal intestine is formed by the pylorus. The anterior and posterior intestines are coated with chitin. The middle intestine is not coated with chitin. Leishmania parasites that develop only in the stomach [the Leishmania (Leishmania) group] are sometimes known as the Suprapylaria. Those that develop on both sides of the pylorus are the Peripylaria [the Leishmania (Viannia) group].