Amebiasis is an infection of the colon caused by a parasitic protozoan, the ameba Entamoeba histoly­tica.

Several species of ameba inhabit the large intes­tine. Most are harmless commensals or minor para­sites, usually causing little or no clinical damage. The closely related species Entamoeba coli and Entamoeba hartmanni are commensals, and infec­tion with E. histolytica is also often asymptomatic. E. histolytica is probably a species complex, with a number of morphologically similar forms with vary­ing degrees of invasiveness. E. hartmanni, formerly believed to be a “small race” of E. histolytica, is now recognized as a separate nonpathogenic species. Pathogenic amebas cause light to severe intestinal damage (amebic dysentery) and sometimes spread to the liver, lungs, brain, and other organs.