"The trophozoites reside in the lumen of the large intestine of humans and animals, where they replicate by binary fission with occasional conjugation.  

Cysts are formed and passed with feces. 

 The cyst is the infectious stage and is acquired by the host through ingestion of contaminated food or water. 

 Following ingestion, excystation occurs in the small intestine, and the trophozoites colonize the large intestine"

• courtesy of the Division of Parasitic Diseases at the National Center for Infectious Diseases, Centers for Disease Control and Prevention