So, one case definition might be:
PERSON: Any tenth-grade student who went on either tour
PLACE: Limited to students at city high schools
TIME: Onset since December 2? 3? 4?
CLINICAL: Confirmed stool sample positive for E. coli O157:H7, regardless of symptoms
SUSPECT: Self-reported severe abdominal pain and diarrhea >2 episodes/day, with stool culture not done; or self-reported abdominal pain and diarrhea >2 episodes/day and stool culture negative