Dysentery is usually caused by a bacterial or protozoan infection or infestation of parasitic worms, but can also be caused by a chemical irritant or virall infection. The most common cause of the disease in developed countries is infection with a bacillus of the Shigella group (causing bacillary dysentery). Infection with the amoeba Entamoeba histolytica can cause amoebic dysentery. This is more common in tropical areas, although has been found throughout the Gulf Coast of the United States, as well as Australia and parts of rural Canada.