Microsporidiosis is a disease caused by any one of several microsporidia organisms, including Enterocytozoon bieneusi. The organisms infect the lining of the small intestine and can cause severe diarrhea and malabsorption (an inability to absorb nutrients). Some microsporidia organisms can also cause sinus, eye, and kidney problems. There have also been reports of microsporidiosis of the lungs.
It is not at all clear how people become infected with microsporidia. It is often found in feces of humans and animals and can be spread via feces-contaminated food and, quite possibly, oral-anal sex ("rimming").
People with compromised immune systems—usually people with a CD4 cell count below 100—may experience prolonged and severe bouts of diarrhea and malabsorption that can be difficult to treat. It's also important to note that not all people exposed to microsporidia, even if their immune systems are suppressed, experience symptoms of the infection.