Acnidosporidia are a subclass of Sporozoa, a group of parasitic protozoans that infect animals and cause diseases. Acnidosporidia include two orders: Sarcosporidia and Haplosporidia, which have different life cycles and hosts.

Sarcosporidia are parasites that form cysts in the muscles of vertebrates, such as mammals, birds, reptiles, and fish. They can cause sarcocystosis, a disease that affects the nervous system and muscles of the infected animals. Some species of Sarcosporidia can also infect humans through the consumption of raw or undercooked meat from infected animals.

Haplosporidia are parasites that infect mainly marine invertebrates, such as mollusks, crustaceans, and annelids. They can cause haplosporidiosis, a disease that affects the tissues and organs of the infected animals. Some species of Haplosporidia can cause mass mortality events in commercially important shellfish, such as oysters and mussels.

Acnidosporidia are distinguished from other Sporozoa by having simple spores that are formed in a manner unlike that of the Telosporidia, another subclass of Sporozoa. The spores of Acnidosporidia have a single-layered wall and contain one or more sporozoites, which are the infective stage of the parasite. The spores also have a characteristic structure called a polar filament, which is used to inject the sporozoites into the host cells.

Acnidosporidia are still poorly understood and their relationship to other Sporozoa is questionable. More research is needed to clarify their taxonomy, phylogeny, biology, and epidemiology.

One of the most studied species of Sarcosporidia is Sarcocystis neurona, which causes equine protozoal myeloencephalitis (EPM) in horses. EPM is a serious neurological disease that can cause weakness, ataxia, paralysis, and death in horses. The definitive hosts of S. neurona are opossums, which shed the spores in their feces. The intermediate hosts are various animals, such as armadillos, raccoons, skunks, and cats, which ingest the spores from the environment and develop cysts in their muscles. Horses become infected by accidentally ingesting the spores from contaminated food or water. The spores then invade the central nervous system and cause inflammation and damage to the brain and spinal cord.

One of the most studied species of Haplosporidia is Haplosporidium nelsoni, which causes MSX disease in eastern oysters. MSX disease is a chronic infection that causes lesions and necrosis in the connective tissue of the oysters. The disease can reduce the growth, reproduction, and survival of the oysters, and can cause significant economic losses for the oyster industry. The life cycle of H. nelsoni is not fully understood, but it is believed that the parasite has two hosts: a mollusk host and an annelid host. The spores are released from the infected oysters and infect the annelids, which serve as vectors for transmitting the parasite to other oysters.