Life cycle of Retortamonas intestinalis
Both cysts and trophozoites of Retortamonas intestinalis are shed in feces (1). Infection occurs after the ingestion of cysts in fecal-contaminated food or water or on fomites (i.e., other non-living objects or substances that can transmit them) (2). In the large (and possibly small) intestine, excystation releases trophozoites. Retortamonas intestinalis resides in the large intestine, where it is regarded as a commensal and is not known to cause disease.
From Centers for Disease Control Parasites and Health website