Schematic life cycle of Leishmania parasites.

The life cycle starts when a parasitized female sandfly takes a blood meal from a vertebrate host (e.g., a human). As the sandfly feeds, infectious promastigote (metacyclic) forms of the parasite enter the vertebrate host. Within the vertebrate host, these forms are phagocytosed by macrophages where they differentiate into amastigote forms. The life cycle is completed when, during a blood meal, a female sandfly ingests infected macrophages. The parasites transform into multiplicative promastigotes inside the sandfly, and after migration into the sandfly's proboscis, promastigotes transform into metacyclic promastigotes (infectious form) and must be delivered to a new host for the life cycle to continue. The possible locations of clonality in the two hosts and of sexual events (recombination between two individuals) in the vector are indicated (figure adapted from http://www.dpd.cdc.gov/dpdx).