Amebiasis, widespread human disease in tropical regions, resulting from infection by the amoeba Entamoeba histolytica. The parasite most commonly is acquired in its encysted form in tainted food or drink. When it only infects the intestines it can cause dysentery, but it may also spread to other organs. Amebiasis is easily dealt with by drugs, but if untreated it can lead to abscesses of the liver, the lungs and, less frequently, the heart; rarely, it may even reach and damage the brain.

Amoeba Engulfing a Paramecium
An amoeba, a single-celled organism lacking internal organs, is shown approaching a much smaller paramecium, which it begins to engulf with large outflowings of its cytoplasm, called pseudopodia. Once the paramecium is completely engulfed, a primitive digestive cavity, called a vacuole, forms around it. In the vacuole, acids break the paramecium down into chemicals that the amoeba can diffuse back into its cytoplasm for nourishment.
Peter Parks/Oxford Scientific Films

Amoeba, any of a group of unicellular organisms characterized by their locomotive method of extending cytoplasm outward to form pseudopodia (false feet). The amoeboid group includes hundreds of different organisms, ranging in size from about .25 to 2.5 mm (about 0.0098 to 0.098 in). Amoebas are considered the most primitive animals and are classified in the kingdom Protista. All amoeboid organisms have thin cell membranes, a semirigid layer of ectoplasm, a granular, jellylike endoplasm, and an oval nucleus. Some species live on aquatic plants and some in moist ground; others are parasitic in animals.

Amoebas also use pseudopodia for feeding. Chemical stimuli from smaller organisms, the amoeba's food, induce the formation of pseudopodia, pairs of which envelop the organism, at the same time forming a cavity, or vacuole. A digestive enzyme secreted into the cavity breaks down this food into soluble chemical substances that then diffuse from the cavity into the cytoplasm. Undigested food and wastes are excreted through the ectoplasm, which also absorbs oxygen from the surrounding water and eliminates carbon dioxide, a by-product of metabolism, in a form of respiration. After a period of growth, the amoeba reproduces by splitting into two equal parts.

At least six forms of amoeba are parasitic in humans. Most important of these is Entamoeba histolytica, which causes amebiasis and dysentery. The diseases often occur in epidemics when raw sewage contaminates water supplies or when soil is fertilized with untreated human wastes.

Dysentery, acute or chronic disease of the large intestine of humans, characterized by frequent passage of small, watery stools, often containing blood and mucus, accompanied by severe abdominal cramps. Ulceration of the walls of the intestine may occur. Although many severe cases of diarrhea have been called dysentery, the word properly refers to a disease caused by either a specific amoeba, Entamoeba histolytica, or a bacillus that infects the colon.

Amoebic dysentery, caused by the parasite Entamoeba histolytica, is endemic in many tropical countries, but is attributable more to unsanitary conditions than to heat. It is the most common type of dysentery in the Philippine Islands, the Malay Archipelago, and the West Indies, but it also occurs in almost all temperate countries. Amoebic dysentery is most commonly spread by water or contaminated, uncooked food or from carriers. Flies may carry the cysts to spread the amoeba from the feces of infected persons to food. Various drugs, including metronidazole, ementine, and iodine-containing preparations, have been useful in treating severe cases of the disease.