Blastocystis species are organisms with worldwide distribution that are classified as similar to protist organisms but are also known to have genomic similarity to Entamoeba histolytica.

Whether or not Blastocystis species cause human disease is controversial. Case reports and case series have associated Blastocystis species with symptoms of diarrhea (acute or chronic) and other nonspecific symptoms, including bloating and abdominal cramps.

Stool can be examined for presence of the parasite. Diagnosis can be made using the polymerase chain reaction, but this test may not be routinely available. Optimal therapy is not known.