Leishmania protozoa. Light micrograph of Leishmania donovani, a parasitic protozoan that causes visceral leishmaniasis (kala-azar). There are two main forms of L. donovani, one that lives in humans and one that is found in its sand fly host. These are the flagellated promastigote form which occurs in sand flies. The parasite passes between the two hosts when sand flies feed on human blood, changing forms once it is inside its new host. Without drug treatment, kala-azar can be fatal. Brightfield illumination. Magnification: x100 at 35 mm size.