Shigellosis: Epidemic and opportunistic (causes disease when the immune system is suppressed) dysentery that is due to infection with shigella bacteria. Shigellosis causes intestinal pain and diarrhea, with mucus and blood in the stool. It is especially common in tropical countries but frequently occurs elsewhere. It is a particular hazard for people with AIDS or other immunodeficiency states. Treatment is with antibiotics against the shigella bacteria.