Our approach has two limitations. First, we assume that all diarrhoea patients with stool specimens containing ipaH are infected with Shigella species. It seems unlikely that ipaH at this site is derived from organisms other than Shigella species, but the presence of Shigella could indicate colonization and not infection. Studies using the same laboratory methods failed to detectipaHinhealthyvolunteersfromBangkok (O.Sethabutret al., unpublished data, 2003) which makes colonization without