These are primarily parasites of terrestrial birds and mammals. The host ingests a sporulated oocyst. Within the intestine 8 sporozoites are released from the oocyst. Sporozoites penetrate epithelial cells in the gastrointestinal tract at a species-specific location. The sporozoite becomes a schizont, in which nuclear division followed by cytokinesis produces a large number of merozoites. The infected cell ruptures and each merozoite penetrates a new epithelial cell.
The process of schizogony repeats 1-4 times, depending upon the species involved. Then, following penetration of a new epithelial cell, the merozoite develops into either a macrogametocyte or a microgametocyte. Microgametocytes produce microgametes that burst from the infected cell, enter cells containing macrogametes, and fertilize them. The resulting zygotes lay down an impervious cyst wall around themselves and are now oocysts.
Oocysts rupture from the host cell and are excreted in the host's waste. Initially, the oocyte is unsporulated, containing only a zygote that is not yet infective to a new host. After a few days, 8 sporozoites are formed within the oocyst. It is now sporulated and infective.
Infections with Eimeria are self-limiting and the host develops some degree of immunity against reinfection.