Introduction: the ˈmicrosporidial worldˈ, a paragon for analyzing intracellular parasitism

Microsporidia are unicellular eukaryotes which share a common status of obligate intracellular parasite. Before gaining strong evidence for their involvement in some human diseases, especially through the rather recent recognition of several microsporidian species as opportunistic pathogens of AIDS patients, they were among the least investigated parasites. This is all the more paradoxical as the phylum Microspora includes more than 1 100 species distributed among about 145 genera. It should be stressed that members of most classes of the kingdom Animalia can be infected by microsporidia. Hyperparasitism cases have been reported in some protozoans (Apicomplexa) as well as in Myxosporidia, Trematoda and insects. To emphasize the worldwide distribution of parasites able to successfully invade various hosts in terrestrial, freshwater and marine environments, I chose to use here the term 'microsporidial world'.