Toxoplasma, Plasmodium, Cryptosporidium, Cylospora The motile stage of a protozoan's life is called the trophozoite stage. The trophozoite stage is usually the form that causes harm to the host. Usually there is an alternative stage in the life cycle of protozoan parasites, the cyst stage. The cyst stage is a non motile stage where the organism is more protected from the environment and usually more difficult to kill. The cyst stage is often the infectious stage , meaning the form the parasite is in when it gets into the human body.
Cysts are how protozoans survive in the environment outside the host's body.
Treatment for a bacterial diarrhea will be different since drugs effective on procaryotes will not usually work on eucaryotes. There are few drugs for protozoan and helminth infections. Entamoeba histolytica has an alternating trophozoite and cyst stage. The cyst can survive in the environment and be infectious to humans. Humans are the primary hosts for Entamoeba.