Giardia (Lamblia) intestinalis, discovered by Lambl in 1859, for a long time was considered a nonpathogenic parasite. It was believed to be rare in temperate climates and was regarded as an organism of negligible importance. Increased interest in this flagellate was awakened during the Great War, as numerous cases of intractable diarrhea were reported which were believed to be caused by this parasite. The English and the French, especially, believed it to be the etiologic agent in many cases of "trench diarrhea." Since the parasite had received but scant attention in the past, many interesting problems regarding its geographic distribution, pathogenicity and treatment presented themselves.
Giardia intestinalis differs from the other protozoa that inhabit the human intestinal tract in that its habitat is the duodenum and jejunum. The vegetative forms are readily detected in the duodenal contents in many cases in which the stools contain few or none of the