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Background: On May 6, 2011, we received a report of an outbreak of gastroenteritis in a rural boarding primary school.We undertook an epidemiological investigation to identify risk factors for illness, recommend prevention measures, and prevent future outbreaks.Methods: We implemented a cross-sectional survey and a case-control study.A case was defined as onset of diarrhea (≧3 times/24 hours) from May 1 to 10 in a student or staff of the school.A control-person was a student or staff without any symptoms during the same period.Results: We identified 26 cases, attack rate: 9.1%, all among boarding students in grades 3 to 6.Most common symptoms were: abdominal pain (92%) and diarrhea (88%), while 8% reported fever.The epidemic curve showed a single peak and an interval of 5 hours between the onsets of the first and last cases.Accordingly, we focused the investigation on at breakfast 4 hours before and dinner 16 hours before the outbreak peak.The study showed 77% of case-students ate >3 steamed buns compared to 32% of control-students (odds ratio [OR]=7.1; 95% confidence interval [CI] =2.0-26).OR increased by 2.4 for each steamed bun eaten (x2 for trend=10.6, P<0.01).The steamed buns were cooked 12 hours before breakfast, stored at room temperature and inadequately reheated before breakfast.Laboratory tests of leftover buns did not include Bacillus cereus.Conclusions: Steamed buns, improperly handled, caused the outbreak.We suspected from the type of food, the symptoms, and nature of mishandling that B.cereus was the responsible pathogen.We recommended the school canteen have an additional refrigerator reserved for cooked food.