(Plural: Protozoa) Any one of a large group of one-celled (unicellular) animals, including amoebas. They are microorganisms that differ from bacteria in that they are larger and possess a nucleus surrounded by a membrane. Several species of protozoa can be transmitted through water and cause disease in humans, including Giardia, Cryptosporidium, Cyclospora, Entamoeba and Isospora. One distinguishing characteristic of protozoa is that when released from the human body through feces they are present in an encysted (dormant) form. These cysts have a protective layer that surrounds them and keeps chemicals from penetrating them. Therefore, chlorine disinfection does not kill the protozoan cysts.