Question 13 During 1999-2000, an outbreak of giardiasis occurred in 69 children aged 5 years and younger in Auckland, New Zealand. Table 1 summarises the behaviours of infected children (cases) and local uninfected children (controls) in the weeks before and during the outbreak. For each behaviour, the odds of developing giardiasis and the odds ratios (OR) were calculated. We have deliberately removed three of the odds ratios from the table. Table 1 Data on risk factors for giardiasis in children studied during an outbreak. Risk factor Case Control Odds OR (n=69) (n =98) Water sources Only drank home mains water 16 59 0.27 1.00 Drank roof-collected rain water 19 12 1.58 5.85 Drank river water 5 1 5.00 18.52 Drank well water 3 1 3.00 11.11 Drank bottled water 7 14 0.50 ? Swimming No swimming 24 51 0.47 1.00 Swam less than once per week 10 15 0.67 143 Swam more than once per week 34 27 1.26 2.68 Swam in swimming pool 29 36 0.81 1.72 Swam in river 6 1 6.00 12.77 Swam in sea 9 5 1.80 ? Toilet Child not in nappy 10 44 0.23 1.00 Child in nappy 59 54 1.09 ? Child partly toilet trained 12 19 0.63 2.74 Child fully toilet trained 12 45 0.27 1.17 Play scheme Does not attend play scheme 27 38 0.71 1.00 Attended, did not eat meal 10 43 0.23 0.32