Giardia intestinalis is an intestinal parasite of humans and other mammals that causes intestinal ailments in most people. It can infect people when they drink water contaminated with feces containing Giardia cysts. Upon ingestion, each cyst releases two motile cells, called trophozoites. These attach to the small intestine's lining via a ventral adhesive disk. The trophozoites anaerobically metabolize glucose from the host's intestinal contents to produce ATP. Reproduction is completely asexual, occurring by longitudinal binary fission of trophozoites, with each daughter cell receiving two haploid nuclei (n = 5). A trophozoite will often encyst as it passes into the large intestine by secreting around itself a case that is resistant to cold, heat, and dehydration.

The primary treatment for giardiasis (infection with Giardia), trichomoniasis (infection with Trichomonas vaginalis), and amoebic dysentery (infection with Entamoeba histolytica) is a drug marketed as Flagyl (generic name is metronidazole). The drug also kills anaerobic gut bacteria. Consequently, which of these are cues that Flagyl's mode of action has nothing to do with attacking or disabling flagella, as the drug's name might imply?

1. Flagyl would also harm the flagellated lining of the human intestine.
2. Entamoeba possesses pseudopods, not flagella, yet it is killed by Flagyl.
3. Not all anaerobic gut bacteria possess flagella, yet flagyl kills these bacteria.