Microsporidiosis: a disease caused by small parasites known as Microsporidia. These parasites typically do not cause problems in healthy people, but may cause disease in people with suppressed immune function. Microsporidiosis can cause intestinal problems such as chronic diarrhea, kidney disease, and infection of the sinuses and eyes, including microsporidia keratitis. Symptoms depend upon the area of the body that is infected. Microsporidia spores are released from the stool, respiratory secretions, and urine of infected animals (insects, birds, and mammals) and humans. The process by which the parasite is transmitted among humans is not completely understood, but ingestion or inhalation of the spores is a likely cause.