Infection caused by a cosmopolitan flagellated protozoan parasite (Giardia lamblia). In some countries more than 10 per cent of the population carry it, and it is particularly widespread among children in the tropics. Europeans who have visited the tropics may also become infested. Giardia lamblia is able to travel through the intestine and it attaches itself to the wall of the duodenum or the small intestine with a sucker, and lives on the liquid contents of the intestine. The parasite can be surrounded by a solid wall (cyst) and be passed with the faeces, and can thus find its way into food and drinking water. The parasite is highly resistant to chlorine, so that even apparently pure water can be a source of infection. Many infected patients show no symptoms, but in more serious cases the small intestine is coated with parasites, causing abdominal pain, nausea, loss of appetite and diarrhoea. Sometimes incomplete digestion of fat leads to malabsorption. The disease can be confirmed by examining faeces for cysts of the parasite. Treatment by drugs is possible.