The microsporidia constitute a phylum (Microspora) of spore-forming unicellular parasites. They were once thought to be protists but are now known to be 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.