Blastocystosis: Infection with Blastocystis hominis, a common microscopic parasite found worldwide. Symptoms may include watery or loose stools, diarrhea, abdominal pain, anal itching, weight loss, and excess gas. Some people have no symptoms. The organism can remain in the intestines for weeks, months, or years. Diagnosis requires finding it in the stool, How Blastocystis is transmitted is unknown, although the number of people infected seems to increase in areas where sanitation and personal hygiene is inadequate. Treatment is with the antibiotics metronidazole or iodoquinol.