Giardia lamblia . Anton Van Leeuwenhoek first identified Giardia in 1681, when he examined one of his own stools with a home-made lens. It took over 200 years, however, for scientists to demonstrate that Giardia is a true enteric pathogen rather than a commensal organism. Giardiasis occurs throughout temperate and tropical regions worldwide, and is one of the most common human protozoal enteric pathogens. It is the most clinically significant protozoan pathogen in the United States. The overall prevalence rate is 2–7%, but reaches up to 35% within day-care centers. Risk factors for infection include pediatric age group, travel to foreign countries, use of drinking and recreational water from untreated sources, institutional settings (including day-care centers), living with an infected child, and immune compromise, particularly common variable immunodeficiency.