[1] Parasites enter the bird host in a form called a sporozoite through the bite of the blood-sucking black fly.

After several days the oocyst produces ~100 sporozoites that leave and migrate to the salivary glands of the fly.

It has been suggested that this genus arose after the two others genera Plasmodium and Haemoproteus, the latter originating from the late Oligocene or early Eocene at about the same time as Piciformes and Coraciiformes.

The oocysts are small and nonexpanding, reaching 13 micrometres in diameter and typically have less than 100 short, thick sporozoites.

Type species: Leucocytozoon ziemanni The typical pathology of infection with these parasites includes anaemia and enlargement of the liver and spleen.

Megaloschizonts appear as grey-white nodules found in the heart, liver, lung or spleen.

Microscopically there is ischemic necrosis and associated inflammation in the heart, brain, spleen and liver due to occlusion of blood vessels by megaloschizonts in endothelial cells.

The excess mortality due to Leucocytozoon in adult birds seems to occur as a result of debilitation and increased susceptibility to secondary infection.

Bird hosts Like many protist species and genera this genus is subject to ongoing revision especially in the light of DNA based taxonomy.

[20] A more detailed description of parasites resembling Leucocytozoon was published in 1894 by N. Sakharoff from the blood of birds near Tbilisi.

[21] Soon thereafter in 1898, Hans Ziemann described a Leucocytozoon parasite from the blood of the owl Athene noctua, naming it Leukocytozoen danilewskyi in honor of Danilewsky.

[21] The genus was subsequently formally defined in 1908 by Louis Sambon, and has remained largely unchanged since.

[21] In 1930 and 1931, Earl O'Roke and Louis V. Skidmore independently discovered black flies to be the vector of Leucocytozoon species.