Microsporidia

The microsporidia constitute a phylum 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. The distinguished species of microsporidia usually infect one specific host or a related group of hosts. Several species, most of which are opportunistic, also infect humans.

Approximately 10 percent of the species are parasites of vertebrates, including humans.

After infection they influence their hosts in various ways and all organs and tissues are invaded, though generally by different species of microsporidia. Some species are lethal, and a few are used in biological control of insect pests. Parasitic castration, gigantism, or change of host sex are all potential effects of microsporidian parasitism (in insects). In the most advanced cases of parasitism the microsporidium rules the host cell completely and controls its metabolism and reproduction, forming a xenoma. .

Replication takes place within the host's cells, which are infected by means of unicellular spores. These vary from 1-40 μm, making them some of the smallest eukaryotes. Microsporidia that infect mammals are 1.0-4.0 μm. They also have the smallest eukaryotic genomes.

Microsporidium was once the vernacular name for a member of the class Microsporidea in the protozoan subphylum Cnidospora.