The term microsporidia is also used as a general nomenclature for the obligate intracellular parasites belonging to the phylum Microsporidia.  To date, more than 1,200 species belonging to 143 genera have been described as parasites infecting a wide range of vertebrate and invertebrate hosts.  Microsporidia, are characterized by the production of resistant spores that vary in size, depending on the species.  They possess a unique organelle, the polar tubule or polar filament, which is coiled inside the spore as demonstrated by its ultrastructure.  The microsporidia spores of species associated with human infection measure from 1 to 4 µm and that is a useful diagnostic feature.  There are at least 15 microsporidian species that have been identified as human pathogens: Anncaliia (formerly Brachiola) algerae, A. connori, A. vesicularum, Encephalitozoon cuniculi, E. hellem, E. intestinalis, Enterocytozoon bieneusi Microsporidium ceylonensis, M. africanum, Nosema ocularum, Pleistophora sp., Trachipleistophora hominis, T. anthropophthera, Vittaforma corneae, and Tubulinosema acridophagus.  Encephalitozoon intestinalis was previously named Septata intestinalis, but it was reclassified as Encephalitozoon intestinalis based on its similarity at the morphologic, antigenic, and molecular levels to other species of this genus.  Based on recent data it is now known that some domestic and wild animals may be naturally infected with the following microsporidian species: E. cuniculi, E. intestinalis, E. bieneusi.  Birds, especially parrots (parakeets, love birds, budgies) are naturally infected with E. hellem.  E. bieneusi and V. corneae have been identified in surface waters, and spores of Nosema sp. (likely A. algerae) have been identified in ditch water. Tubulinosema acridophagus, an insect parasite, has recently (2012) been implicated in two cases of disseminated microsporidiosis.