Caused by a parasite (Giardia lamblia) responsible for a contagious form of diarrhea. The parasite is most commonly transmitted through direct contact with infected feces or by eating food or drinking water contaminated by feces. Giardia is one of the most common intestinal parasites in the world. The disease is most prevalent in developing countries, where infections are associated with poor sanitary conditions, poor water quality control, and overcrowding. Giardia is also a major cause of waterborne outbreaks of diarrhea in the U.S., primarily in mountainous areas where water supplies may be contaminated with feces from humans or animals. Campers and backpackers should avoid drinking untreated water from mountain streams. Giardiasis affects three times as many children as adults. It particularly affects diapered children and toddlers being toilet-trained. Families with young children who attend day-care centers are at greater risk of developing giardiasis than is the general population. The parasite lives in two stages: trophozoites and cysts. Trophozoites are the active form of the parasite inside the body. Cysts represent the resting stage that enables the parasite to survive outside the body. Infection begins from swallowing the Giardia cysts. The acid in the stomach activates the cysts, which in turn release the trophozoites. The parasites attach to the lining of the small intestine, reproduce, and are swept down the intestine in the fecal stream. Cysts form in the lower intestines and are then passed in the feces. The symptoms of giardiasis include: a sudden explosive, watery, foul-smelling diarrhea; excessive gas; abdominal pain; bloating; nausea; tiredness; and loss of appetite. In some patients, vomiting is the major symptom. Fever is unusual. If not treated, giardiasis can last for months, or even years. The illness can cause recurrent mild or moderate symptoms such as impaired digestion, especially lactose intolerance; intermittent diarrhea; fatigue, weakness, and significant weight loss. For diagnosis, stool specimens are collected and examined over a period of at least 3 days. The diagnosis is made by microscopic identification of the parasite in stool. Tests that detect antigens (proteins) to Giardia in the feces are especially useful for screening children in day-care settings, and for testing adults after treatment. Treatment may be with metronidazole (Flagyl). Pregnant women are advised not to take metronidazole during the first trimester of pregnancy. Paromomycin is an alternative and safer drug. If relapses occur, re-treatment with the same drug is usually effective. Furazolidone (Furoxone) may be given to infants and to children under 5 years old, as it comes in a liquid form.