Giardiasis: Giardiasis is an infection of the small bowel caused by Giardia lamblia. It is the most common intestinal parasitic disease in the U.S. Causes of giardiasis include contaminated water and food supplies, and person to person contact. Symptoms of giardiasis include diarrhea, weight loss, bloating, gas, constipation, nausea, vomiting, rash, and fever. Treatment of giardiasis is generally with antibiotics.