"Infection occurs by the ingestion of cysts in contaminated water or food. 

In the small intestine, excystation releases trophozoites that multiply by longitudinal binary fission. 

The trophozoites remain in the lumen of the proximal small bowel where they can be free or attached to the mucosa by a ventral sucking disk. 

Encystation occurs when the parasites transit toward the colon, and cysts are the stage found in normal (non diarrheal) feces. 

 The cysts are hardy, can survive several months in cold water, and are responsible for transmission. 

Because the cysts are infectious when passed in the stool or shortly afterward, person-to-person transmission is possible. 

While animals are infected with Giardia, their importance as a reservoir is unclear."

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