Entamoeba histolytica is an anaerobic parasitic protozoan that causes amoebiasis, presenting with either bloody diarrhea or liver abscesses with an "anchovy paste" exudate. Histology shows flask-shaped ulcers in gastrointestinal submucosa. Transmission is through cysts in water. Diagnosis is made either with serology, or with stool examination showing trophozoites or cysts in it. Metronidazole is the drug of choice; iodoquinol is used for asymptomatic carriers who pass cysts in their stool.