Causes and symptoms
Recently, it has been discovered that persons with symptom-causing amebiasis are infected with Entamoeba histolytica, and those individuals  who exhibit no symptoms  are actually infected with an almost identical-looking ameba called Entamoeba dispar. During their life cycles, the amebas exist in two very different forms: the infective cyst or capsuled form, which cannot move but can survive outside the human body because of its protective covering, and the disease-producing form, the trophozoite, which although capable of moving, cannot survive once excreted in the feces and, therefore, cannot infect others. The disease is most commonly transmitted when a person eats food or drinks water containing  E. histolytica cysts  from human feces. In  the digestive tract the cysts are transported to the intestine where the walls of the cysts are broken open by digestive secretions, releasing the mobile trophozoites. Once released within the intestine, the trophozoites multiply by feeding on intestinal bacteria or by invading the lining of the large intestine. Within the lining of the large intestine, the trophozoites secrete a substance that destroys intestinal tissue and creates a distinctive bottle-shaped sore (ulcer).  The trophozoites may remain inside the intestine, in the intestinal wall, or may break through the intestinal wall and be carried by the blood to the liver, lungs, brain, or other organs. Trophozoites that remain in the intestines eventually form new cysts that  are carried through the digestive tract  and excreted in the feces. Under favorable temperature and humidity conditions, the cysts  can survive in soil or water for weeks to months, ready to begin the cycle again. 

Although 90% of cases of amebiasis in the United States are mild, pregnant women, children under two years of age, the elderly, malnourished individuals,  and people whose immune systems may be compressed, such as cancer or AIDS patients and those individuals taking prescription medications that suppress the immune system, are at  a greater risk for developing a  severe infection. 

The signs and symptoms  of amebiasis vary according to the location and severity of the infection and are classified as follows: 

Intestinal amebiasis 

Intestinal amebiasis can be subdivided into several categories: 

• ASYMPTOMATIC INFECTION.  Most persons  with amebiasis have no noticeable symptoms. Even though these individuals may not feel ill, they are still capable of infecting others by person-to-person contact or by contaminating food or water with cysts that others may ingest, for example, by preparing food with unwashed hands. 
• CHRONIC NON-DYSENTERIC INFECTION.  Individuals may experience symptoms over a long period of time during a chronic amebiasis infection  and experience recurrent episodes of diarrhea that last from one to four weeks and recur over a period of years. These patients may also suffer from abdominal cramps, fatigue, and weight loss. 
• AMEBIC DYSENTERY.  In severe cases of intestinal amebiasis, the organism  invades the lining  of the intestine, producing sores (ulcers), bloody diarrhea, severe abdominal cramps, vomiting, chills, and fevers  as high as 104-105°F (40-40.6°C). In addition, a case of acute amebic dysentery may cause complications, including inflammation of  the appendix (appendicitis), a tear in the intestinal wall (perforation),  or a sudden, severe inflammation of the colon (fulminating colitis). 
• AMEBOMA. An ameboma is a mass of tissue in the bowel that is formed by the amebiasis organism. It can result from either chronic intestinal infection or acute amebic dysentery. Amebomas may produce symptoms that mimic cancer or other intestinal diseases. 
• PERIANAL ULCERS.  Intestinal amebiasis may produce skin infections in the area around the patient's anus (perianal). These ulcerated areas have a "punched-out" appearance and are painful to the touch. 

Extraintestinal amebiasis 

Extraintestinal amebiasis accounts for approximately 10% of all reported amebiasis cases and includes all forms of the disease that affect other organs. 

The most common form of extraintestinal amebiasis is amebic abscess of the liver. In the United States, amebic liver abscesses occur most frequently in young Hispanic adults. An amebic liver abscess can result from direct infection of the liver by E. histolytica or as a complication of intestinal amebiasis. Patients with an amebic abscess of the liver complain of pain in the chest or abdomen, fever, nausea, and tenderness on the right side directly above the liver. 

Other forms of extraintestinal amebiasis, though rare, include infections of the lungs, chest cavity, brain, or genitals. These are extremely serious and have a relatively high mortality rate.