A large group of protozoans that are characterized by having a special organelle called an apical complex. Most of them are single-celled, parasitic, and spore-forming.
This group includes organisms such as coccidian, gregarines, piroplasm, haemogregarines, and malarias. The first apicomplexan protozoon observed was the Eimeria stiedai oocyst in the gall bladder of a rabbit by Anton van Leeuwenhoek in 1674.
Apicomplexans are sometimes taken as synonymous with sporozoans. At other times sporozoans are regarded as only a subset of the apicomplexans.
See also: apical complex, Sprozoan