Giardia protozoan. Coloured scanning electron micrograph (SEM) of a Giardia sp. trophozoite protozoan (yellow) attached to the lining of a human intestine. Giardia are single-celled parasites of the intestinal tract. They are most common in tropical regions and have flagella (hair-like structures), which are used for locomotion. They are the causative agent of giardiasis, an intestinal disease that causes abdominal pain, diarrhoea and nausea. It is spread through contaminated food and water. Treatment is with antibiotics or antiprotozoal drugs, such as metronidazole.