Giardia ( /dʒiːˈɑrdiə/ or /ˈdʒɑrdiə/) is a genus of anaerobic flagellated protozoan parasites of the phylum Diplomonada in the supergroup "Excavata" (named for the excavated groove on one side of the cell body) that colonise and reproduce in the small intestines of several vertebrates, causing giardiasis. Their life cycle alternates between an actively swimming trophozoite and an infective, resistant cyst. The genus was named after French zoologist Alfred Mathieu Giard.
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