Giardia is a genus of anaerobic flagellated protozoan parasites of the phylum Sarcomastigophora that colonise and reproduce in the small intestines of several vertebrates, causing giardiasis. Their life cycle alternates between a swimming trophozoite and an infective, resistant cyst. Giardia were first described by the Dutch microscopist Antonie van Leeuwenhoek in 1681. The genus is named after French zoologist Alfred Mathieu Giard.