The sandflies become infected during blood meals on an infected host when they ingest macrophages infected with amastigotes. In the sandfly, the parasites differentiate into promastigotes that multiply and mature. A minimum of 7 days are required before infective promastigotes are present. The promastigotes block the insect's pharynx and hypostome, and are regurgitated by the sandfly during a subsequent blood meal. Promastigotes that reach the puncture wound and are phagocytized by macrophages transform into amastigotes and multiply, thus closing the life cycle.
Multiplication is by binary fission, and no sexual stage has been identified.
Approximately 350 million people live in these areas. Most of the affected countries are in the tropics and subtropics. The settings in which leishmaniasis is found range from rain forests in Central and South America to deserts in West Asia. More than 90 percent of the world's cases of visceral leishmaniasis are in India, Bangladesh, Nepal, Sudan, and Brazil.