Following an unusually heavy rainfall in June 2009, a community-wide outbreak of Campylobacter gastroenteritis occurred in a small Danish town. The outbreak investigation consisted of (1) a cohort study using an e-questionnaire of disease determinants, (2) microbiological study of stool samples, (3) serological study of blood samples from cases and asymptomatic members of case households, and (4) environmental analyses of the water distribution system. The questionnaire study identified 163 cases (respondent attack rate 16%). Results showed a significant dose-response relationship between consumption of tap water and risk of gastroenteritis. Campylobacter jejuni belonging to two related aA types were isolated from stool samples. Serum antibody levels against Campylobacter were significantly higher in cases than in asymptomatic persons. Water samples were positive for coliform bacteria, and the likely mode of contamination was found to be surface water leaking into the drinking-water system. In the week beginning 15 June 2009, general practitioners (GPs) in the town of Tune, 30 km south of Copenhagen, noticed an unusual increase in patients with acute gastroenteritis. Campylobacter spp. were detected in stool cultures from some patients. The suspicion of a waterborne gastroenteritis outbreak was raised, an outbreak investigation began and a tap-water boiling advice was issued on 20 June. In the following, we present the outbreak analysis, including a serological follow-up study of persons affected by this waterborne Campylobacter outbreak. A cohort study was initiated using an online questionnaire with questions about signs and symptoms, onset date, duration of illness and exposure to potential sources of Campylobacter infection (e.g. consumption of unboiled tap water, poultry or raw milk, eating ready-to-eat meals from local shops, etc.). A clinical case was defined as a person with diarrhoea (>3 loose stools in 24 h) or abdominal pain with either fever or vomiting, with symptom onset during the period 13-26 June 2009. A confirmed case was defined as a clinical case with a stool culture positive for Campylobacter spp. Water samples were repeatedly taken at the waterworks and at multiple points of the water pipe network following the suspected contamination event. The questionnaire survey was completed by a total of 1039 individuals, response rate 21%, (61% females, 39% males) ranging from 1 to 86 years (mean age 42 years). Of the remaining 933 persons, 159 (153%) were classified as cases and 774 as non-cases, based on their reported symptoms and onset dates. Of these, 41 were culture-positive for Campylobacter jejuni. Cultures for other enteric bacteria were negative. Virological investigations detected sapovirus in two (10%) out of 20 samples tested, and one patient tested positive for both sapovirus and C. jejuni. During 20 June to 30 July, a boiling order was in place for the whole town and the inhabitants were supplied with drinking water from tank trucks. The technical assessment revealed a faulty installation adjacent to one of the boreholes which may have allowed a backflow of sewage water into the gravel surrounding the borehole, when the sewage system was congested due to the heavy rainfall on 12 June. The outbreak was most likely caused by drinking water as shown by several lines of evidence. Second, an exceptionally heavy rainfall occurred a few days before the outbreak, leading to a drinking water contamination caused by congestion of the combined rainwater drainage and sewage system. ailable at 

{'answer_start': [3129], 'text': ['backflow of sewage water ']}