Sarcocystosis is caused by species of Sarcocystis, an intracellular protozoan parasite in the phylum Apicomplexa. These parasites have an indirect life cycle, cycling between a definitive and an intermediate host. Intestinal infections occur in the definitive host, and tissue invasion is seen in the intermediate host. More than a hundred species of Sarcocystis are parasites of domestic and wild animals. Many of these infections are asymptomatic, particularly in the definitive host.

Equine protozoal myeloencephalitis is usually caused by Sarcocystis neurona. Neospora caninum and/or Neospora hughesi, not discussed in this outline, have also been implicated in some cases. Symptomatic infections caused by Sarcocystis species are reported occasionally in other domestic animals.

Humans are a definitive host for S. suihominis, found in pork, and S. hominis, found in beef. These parasites infect the intestines. Humans can be intermediate hosts for a variety of other Sarcocystis species. The parasites are found in the muscles. The species of Sarcocystis involved is, in many cases, unknown.