Over 500 species are in this order, organised into four families: the Garniidae, the Haemoproteidae, the Leucocytozoidae, and the Plasmodiidae.

Several genera infect mammals: Bioccala, Biguetiella, Dionisia, Hepatocystis, Plasmodium, Polychromophilus, Nycteria, and Rayella.

The insect vectors of Hepatocystis, Plasmodium and Polychromophilus are Ceratopogonidae, Culicidae, and Nycteribiidae, respectively.

[4][5] Pirhemocyton although once thought to be a protozoan has since been shown to be intraerythrocytic inclusion bodies due to a viral infection.

Morrison has shown using molecular data that the Haemosporidia are nested within the gregarines and that this clade is distinct from the piroplasms.