Giardia is a protozoan disease; i.e., it is caused by a single - celled «animal»
flagellate parasite, so - called because it is highly motile, having a tail.
The most commonly occurring intestinal
parasites are nematodes (ascarids, strongyloides, pentastomes) protozoa (coccidia,
flagellates, cryptosporidia, amoeba) and trematodes (flukes).