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- Giardia lamblia (syn. G. duodenalis or G. intestinalis)
- Life cycle forms are: 1) highly infectious cysts; and 2) disease-causing trophozoites.
- Following excystation, trophozoites multiply and colonize the upper small intestine.
- Trophozoites with flat ventral surface adhere to brush border of enterocytes and cause malabsorption. 
- Transmitted by contaminated water, food, person-to-person, and fecal-oral contact. 
- Surface water is easily contaminated by cysts shed from mammalian hosts such as beaver, sheep, cattle, dogs, or cats.
- Highly contagious, low infective dose of 10 cysts.
- Occurs worldwide, specifically in settings of day care centers, over-crowded areas with poor sanitation, and outdoor recreation or swimming pools.