Giardia is a genus of anaerobic flagellated protozoan parasites of the phylum Metamonada that colonise and reproduce in the small intestines of several vertebrates, causing the disease giardiasis. Their life cycle alternates between a binucleated motile trophozoite and an infective, metabolically inert, environmentally resistant tetranucleate cyst. Cysts are transmitted between hosts through the fecal–oral route, contaminated water and/or food. Giardia were first seen by the Dutch microscopist Antonie van Leeuwenhoek in 1681 under the light microscope. The genus is named after French zoologist Alfred Mathieu Giard.