Apicomplexans are a large group of parasitic protists - primative, single-celled organisms. All apicomplexans are parasites of animals and can form resistant spores. Most apicomplexans possess an apical complex structure, which is a special part of their body that is used to penetrate a host's cell. Some have motile structures such as flagella or pseudopods. Diseases caused by apicomplexan organisms include malaria (Plasmodium), Cryptosporidiosis (Cryptosporidium parvum) and Toxoplasmosis (Toxoplasma gondii).