Cysts are the parasite stage responsible for transmission of balantidiasis .  The host most often acquires the cyst through ingestion of contaminated food or water .  Following ingestion, excystation occurs in the small intestine, and the trophozoites colonize the large intestine .  The trophozoites reside in the lumen of the large intestine of humans and animals, where they replicate by binary fission, during which conjugation may occur .  Trophozoites undergo encystation to produce infective cysts .  Some trophozoites invade the wall of the colon and multiply.  Some return to lumen and disintegrate.  Mature cysts are passed with feces .