This article describes a parasitic fungi group. For the ascomycete fungi genus, see Microsporum. For the green algae genus, see Microspora. For the beetle family, see Microsporidae. The Microsporidia constitute a group of spore-forming unicellular parasites. They were once considered protozoans or protists, but are now known to be fungi, or a sister group to fungi. Loosely 1500 of the probably more than one million species are named now. Microsporidia are restricted to animal hosts, and all major groups of animals host microsporidia. Most infect insects, but they are also responsible for common diseases of crustaceans and fish. The named species of microsporidia usually infect one host species or a group of closely related taxa. Several species, most of which are opportunistic, also infect