The life cycle of the Leishmania protozoan parasite.

Leishmaniasis is transmitted by the bite of female phlebotomine sandflies.  The sandflies inject the infective stage, promastigotes, during blood meals .  Promastigotes that reach the puncture wound are phagocytized by macrophages and transform into amastigotes .  Amastigotes multiply in infected cells and affect different tissues, depending in part on the Leishmania species .  This originates the clinical manifestations of leishmaniasis.  Sandflies become infected during blood meals on an infected host when they ingest macrophages infected with amastigotes (, ).  In the sandfly's midgut, the parasites differentiate into promastigotes , which multiply and migrate to the proboscis .

(The above graph originates from the Parasite Image Library whose text is also reproduced here in a modified form)