Blastocystis is a genus of single-celled parasites belonging to the Stramenopiles that includes algae, diatoms, and water molds.

[5][8] Although the role of Blastocystis hominis in human disease is often referred to as controversial, a systematic survey of research studies conducted by 11 infectious disease specialists from nine countries, found that over 95% of papers published in the 10 years prior identified it as causing illness in immunocompetent individuals.

[7] The paper attributed confusion over pathogenicity to the existence of asymptomatic carriers, a phenomenon the study noted is common to all gastrointestinal protozoa.

The fact that Blastocystis' infection route is oral-anal indicates that carriers have been in contact with faecal contaminated matter which might have included other intestinal pathogens that explain the observed symptoms.

The original description of Blastocystis was as a yeast due to its yeast-like glistening appearance in fresh wet mounts and the absence of pseudopodia and locomotion.

[14] This was then contradicted by Zierdt,[15] who reclassified it under subphylum Sporozoa (and later in Sarcodina), based on some distinctive protistan features[which?]

Its sensitivity to antiprotozoal drugs and its inability to grow on fungal media further indicated that it was a protozoan.

[21] Symptoms associated with the infection are diarrhea, constipation, nausea, abdominal cramps, bloating, excessive gas, and anal itching.

[24] Most cases of the infection appear to become diagnosed as irritable bowel syndrome (IBS), according to studies from Denmark,[25] Pakistan, the United Kingdom, and Italy.

[26] In the early 2000s, Egyptian physicians identified 84 patients with diarrhea and enteritis apparently caused by Blastocystis hominis.

After three days of nitazoxanide treatment, symptoms cleared and no fecal organisms were detectable in 36 (86%) of 42 treated patients and in 16 (38%) of 42 people who received placebo (P < .0001).

[35] Conclusively stating that Blastocystis has an animal reservoir depends upon unraveling the true nature of its transmission.

If, as Noël et al. deem likely based upon their own molecular work and a review of the literature, animal-to-human transmission is possible, then animals such as pigs and dogs could in fact be acting as a large reservoir capable of human infection.

[36] Epidemiological studies finding that infection is more common in people living in proximity to farm animals or pets[26] further supports this notion.

Other functions, such as cell division during reproduction and the deposition of apoptotic bodies, have been proposed, although more tests need to be done to validate these roles.

Additionally, it suggested the symptoms could be due to the accumulation of the strongly adhesive amoeboid forms on the host's intestinal wall.

[37] The Blastocystis cyst form is a more recent discovery and has helped in the advancement of understanding the way the infection is transmitted.

Besides, the cysts did not lyse when placed in distilled water and could survive well at room temperature for up to 19 days, indicating its strong resistance.

Researchers have reported that patients with irritable bowel syndrome may provide a reliable source for xenic Blastocystis isolates.