Gastrointestinal amebiasis is an infection of the large intestine caused by microscopic one-celled parasites commonly known as amoebas (Entamoeba histolytica). Because these parasites live in the large intestine, they travel in the feces of infected people, and can contaminate water supplies in places where sanitation is poor. The parasite can contaminate fruits and vegetables grown in areas where human feces are used as fertilizer. They can be transferred on the dirty hands of infected people who don't wash their hands often or correctly.
Once amoebas enter the mouth, they travel through the digestive system and settle in the large intestine. Harmless strains of the parasite (Entamoeba dispar) live there without causing damage. E. histolytica can live in the intestine without causing symptoms, but it also can cause severe disease. These amoebas may invade the wall of the intestine, leasing to amoebic dysentery, an illness that causes intestinal ulcers, bleeding, increased mucus production and diarrhea. These amoebas also may pass into the bloodstream and travel to the liver or, infrequently, to the brain, where they form pockets of infection (abscesses).
About 10% of the world's population is infected with amoebas, particularly people who live in Mexico, India, Central America, South America, Africa and the tropical areas of Asia. In industrialized countries, amebiasis is most common in recent immigrants and travelers who visit countries where amoebas are prevalent.