a genus of endocellular parasitic protozoans of the order Myxosporidia. There are more than 90 species of Nosema—according to some sources, about 120 species. The Nosema are animal parasites that attack either certain tissues and organs or the entire body of the host. Some species are economically harmful, infesting silkworms, fish, bees, and commercially valuable invertebrates. Other species, which cause epizootics among crop pests, can be used for the biological control of economically harmful organisms. A typical representative of the genus is Nosema bombycis, a parasite of the Asiatic silkworm; its spores, which are about 8 microns long, are structurally complex.