Intestinal parasites. Illustrations of the parasitic amoeba Entamoeba histolytica (left) and the parasitic protozoan Giardia lamblia (right). E. histolytica causes amoebic dysentery (amoebiasis) and ulcers. Symptoms include bloody diarrhoea, abdominal pain and nausea.It is spread by faecal contamination of food and water and is most common where sanitation is poor. G. lamblia infection, known as giardiasis, causes diarrhoea, abdominal pain and flatulence. It is spread through contaminated water and contact with infected individuals.