Microsporidia infection is caused by parasitic obligate intracellular spore-producing fungi or closely similar organisms. Previously, microsporidia were categorized as protozoa. Of the more than 1,400 species of microsporidia, at least 15 are linked to disease in humans. Inhalation, ingestion, interaction with animals, direct contact with the conjunctiva, and person-to-person transmission are all ways that the spores of the organism might be acquired.
Only a few cases of human infection were documented before the AIDS epidemic, possibly because there was less general knowledge of microsporidia infection. Microsporidia are likely a prevalent source of subclinical or moderate self-limited sickness in otherwise healthy individuals. Microsporidial keratoconjunctivitis in immunocompetent persons has been documented more frequently lately.