OPPORTUNISTIC PARASITES: Microsporidia are primitive eukaryotic parasites widespread in a large range of animal species. These opportunistic parasites can cause infections in humans, mainly in immunocompromised patients.Four microsporidian species are important in human pathology, Enterocytozoon bieneusi, Encephalitozoon intestinalis, Encephalitozoon cuniculi and Encephalitozoon hellem.A difficult task, laboratory diagnosis is based on direct microscope visualization of the parasite. Special stains not used in routine practice are required for identifying spores.Species differentiation, achieved with the polymerase chain reaction technique, is necessary to select the appropriate treatment. Treatment of the most common microsporidiosis (caused by Enterocytozoon bieneusi) with fumagillin is currently under assessment in an ANRS clinical trial.