Giardia lamblia protozoa, coloured scanning electron micrograph (SEM). These are single-celled parasites of the intestinal tract. They are most common in tropical regions. This is the trophozoite (active, feeding) life stage of the protozoa. The cells have flagella (hair-like structures), which are used for locomotion. Giardia lamblia is 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.