Blastocystis hominis was originally classified as a non-pathogenic yeast, but it is now being recognized as a protozoan, even though it does not have the cyst and trophozoite stages typical of protozoa. The disease it causes, blastocystis, infects the region where the small intestine meets the colon. When the organism is found in great enough numbers, it will cause symptoms of nausea, gas, abdominal pain, diarrhea, and malaise. According to the Great Smokies Diagnostic Laboratory, it is the most commonly detected parasite found in stool samples, with infections more numerous in the adult male and the immunocompromised with chronic fatigue. Since this organism can suppress the immune system, its toxic byproduct is being associated more and more with other autoimmune disorders. Although it is rare to find it outside the intestinal tract, it has been found in the fluid of inflammed knee joints, and has been implicated with other weak pathogens found in irritable bowel syndrome, chronic fatigue syndrome, and various forms of arthritis.
Also, different strains are more pathogenic than others. For example, Canadian strains are usually non-pathogenic, while those of some Middle Eastern countries are. Those who are infected with this organism at the same time as Entamoeba species often experience the following symptoms: bloating, diarrhea, flatulence, constipation, cramps, and malabsorption problems. This parasite could be more prevalent, but, because of poor lab techniques, plus the fact that doctors seldom look for parasites, this actual cause is often overlooked.