Entamoeba Histolytica can cause a severe form of dysentery which can be lethal, particularly if it invades the lungs, liver or brain. It has a similar mode of transmission to giardia and can also be caught from contaminated feces. What makes it so dangerous is that it can rapidly become invasive and pathogenic when the body becomes stressed from either physical or psychological problems. There are a large number of species of amoeba that parasitize the human intestinal tract. Of these Entamoeba histolytica (Entamoeba dispar) are the only species found to be associated with intestinal disease. Although many people worldwide harbor this organism, only about 10% develop clinically-invasive disease and thus the parasite exhibits two very differing clinical presentations. The commensal or non-invasive luminal form causes no signs or symptoms of disease; the pathogenic or invasive form invades the intestinal mucosa and produces dysentery and may give rise to extra-intestinal lesions via the blood, mainly to the liver.

The invasive and non-invasive strains of E. histolytica can be differentiated by isoenzyme electrophoresis. The application of molecular biology has confirmed the presence of two distinct species with the same morphological features. The pathogenic (invasive) species has retained the name E. histolytica and the non-pathogenic, non-invasive species has been named E. dispar.