Intracellular spore-forming protozoa that infect many animals and are known to cause human disease, especially in those with AIDS. The genera of microsporidia implicated are Encephalitozoon, Pleistophora, Septata, Nosema, and Enter ocytozoon. They cause a variety of pathological conditions, including diarrhea, wasting, keratoconjunctivitis, peritonitis, myositis, and hepatitis.