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- Protozoan species, genus Entamoeba.
- E. histolytica is morphologically similar to E. dispar, E. moshkovskii, and E. bangladeshi.
- Mature cysts ingested, excystation occurs in the distal ileum. Motile trophozoites are released and migrate to the large intestine. Both trophozoites and cysts are excreted in stool.
- Infecting dose is fewer than 10 cysts. 
- Cysts found in contaminated water or food.
- Occurs primarily in developing countries, also seen in immigrants from and travelers to endemic areas and those with oral-anal or oral-fecal contact.
- Cysts remain viable outside host in water, soil, and sewage for wks or mos.