Blastocystis hominis is an anaerobic protozoan frequently found in the human gastrointestinal tract. There is ongoing controversy within the medical community as to whether B. hominis is even a pathogen--it may be part of the normal bacterial flora present in stool, or it may be a cause of gastrointestinal disease. Some physicians treat patients with B. hominis who are immunocompromised (AIDS, transplant recipients, and cancer patients, for example) without evidence of another infectious cause; others will not treat for it because there is no conclusive evidence to suggest that B. hominis is pathogenic and the asymptomatic carrier state has been well documented.