Giardia is one of the most common intestinal parasites in the world(13). There are estimates that there may be as many as 2.5 million cases each year of Giardia intestinalis in the US alone(6). A single-celled protozoa, the parasite also poses a serious threat abroad, and exists at very high prevalence rates particularly in places with poor water sanitation. Domestically, Giardia is often associated with hikers and backpackers getting diarrhea after drinking untreated water in the wilderness.  In fact, Giardia is also very common in daycare settings (it can be passed fecal-orally) and has been found even in municipal water supplies and caused local epidemics(12).  Giardia affects many animals as well.  While it is unclear whether or not humans can contract the infection from animals, it is clear that human infections are highly contagious from one person to another(1). Giardia intestinalis is very rarely fatal, causing death usually by dehydration or severe weight loss only in infants and young children in developing countries(19). In fact, many victims of Giardia are asymptomatic, and it wasn't known until the 1970s that the protozoa is actually pathogenic.  The infection is easily treatable with antibiotics, though they may have bad side effects and recurrence is common, particularly in the developing world.   The best treatment for Giardia is actually prevention, and educating people not to drink potentially contaminated water, and to use good personal hygiene.

Classification(8)

Kingdom: Protista
Subkingdom: Protazoa
Phylum: Sarcomastigophora
Subphylum: Mastigophora
Class: Zoomastigophora
Order: Diplomonadida
Family: Hexamitidae
Genus: Giardia
Species: lambia/intestinalis (see note -->)       

Synonyms:

Giardia lamblia (most common in scientific literature)
Giardia duodenalis           

Causes Giardiasis, also called “traveler’s diarrhea” or “beaver fever”

Please note:
There has been much debate about Giardia’s classification and nomenclature.  Currently, the scientific literature distinguishes morphologically between six different species of Giardia: G. intestinalis in mammals, G. agilis in amphibians, G. muris in rodents, G. ardeae and G. psittaci in birds, and G. microti in muskrats and voles (18).  However, the morphological differences are slight, and there is still not a clear consensus on exactly where the lines are drawn. 

The debate in classification also spills over into a debate on nomenclature, and which species name is preferable to use. Although Giardia lamblia and Giardia duodenalis are both used in much of the scientific literature, there is quite a bit of literature discussing the relative merits of using Giardia intestinalis over Giardia lamblia. (12)  Also, the WHO now accepts Giardia intestinalis as the proper nomenclature.(15) Based on this precedent, G. intestinalis is the primary name used in this website.