E. hystolitica survives in an amoeboid form in the intestines. Spherical cysts are excreted in the feces. When ingested, an oocyst infects the cells of the digestive tract, epithelium, liver, kidneys, and blood. The entire life cycle occurs intracellularly. Excretion occurs in the feces.
Protozoans pose a hazard primarily in areas lacking sanitary conditions.
Ingestion of and primary contact with water are the primary methods of transmission. Giardia and Cryptosporidium can both infect mammals if the oocysts are ingested. Urban runoff may convey cysts from litter and domestic pet excreta. Periodically, treatment facilities may find it necessary to bypass treatment of their wastewater. In this case, water containing protozoan cysts is discharged directly into the surface water body.