1. People are infected when they are bitten by an infected female sand fly. Sand flies inject a form of the protozoa (called promastigotes) that can cause infection.
2. Promastigotes are ingested by certain immune cells called macrophages. (The process of a cell ingesting a microorganism, another cell, or cell fragments is called phagocytosis, and cells that ingest are called phagocytes.)
3. In these cells, promastigotes develop into another form (called amastigotes).
4. Amastigotes multiply inside macrophages in various tissues.
5–6. When a sand fly bites an infected person or animal, it becomes infected by ingesting blood containing macrophages with amastigotes inside.
7. In the middle part of the fly's gut (midgut), amastigotes develop into promastigotes.
8. In the fly's midgut, promastigotes multiply, develop, and migrate to the fly's mouth parts. They are injected when the fly bites another person, completing the cycle.