A condition caused by a parasitic organism known as Giardia lamblia, which can colonises the duodenum (see INTESTINE) and the upper part of the small intestine. This organism is usually harmless, but is sometimes responsible for an illness, starting one or two weeks after exposure, with explosive diarrhoea, passing pale fatty stools, abdominal pain and nausea. It responds well to METRONIDAZOLE or MEPACRINE.