Leishmaniasis protozoan life-cycle. Artwork showing the life-cycle of the parasitic Leishmania protozoan that causes leishmaniasis in humans. Leishmaniasis is transmitted through the bite of female sand flies, which inject the protozoan's infective stage (promastigotes, 2) into the blood system whilst feeding (1) on human blood. The promastigotes are engulfed by macrophage white blood cells where they transform into amastigotes (3), which multiply, burst out of their host cell (4) and infect other cells in the same way. Sandflies feeding on the infected person ingest amastigotes (5,6), which mature into promastigotes (7) and multiply inside the fly (8), before migrating to the proboscis ready to be injected into a new person.