Signs and symptoms possibly associated with Blastocystis hominis include:

• Watery diarrhea
• Nausea
• Abdominal pain
• Bloating
• Excessive gas (flatulence)
• Loss of appetite
• Fatigue

Blastocystis is a parasite — a microscopic single-celled organism (protozoan). Many protozoans normally live in your gastrointestinal tract and are harmless or even helpful; others cause disease.

It's not clear whether blastocystis causes disease. Most people who carry the organism have no signs or symptoms, but it's also found in people who have diarrhea and other digestive problems. Blastocystis often appears with other organisms, so it's not known whether it causes disease.

Experts suspect that blastocystis gets into the digestive system when people eat contaminated food or are exposed to the stool of a contaminated person, such as when changing a diaper in a child care setting. Rates of the organism in stool increase where there's inadequate sanitation and poor personal hygiene.

Blastocystis hominis is common, and anyone can have the organism in his or her stools. You might be at higher risk if you travel or live where sanitation is inadequate or where the water might not be safe or if you handle contaminated animals, such as pigs and poultry.

If you have diarrhea associated with Blastocystis hominis, it's likely to be self-limiting. However, anytime you have diarrhea, you lose vital fluids, salts and minerals, which can lead to dehydration. Children are especially vulnerable to dehydration.