A fictional school-meal cohort

A fictional school-meal investigation follows all one hundred students who attended an event. Forty ate the supplied salad and twelve of those developed the defined illness during the same follow-up period. Sixty did not eat it and six of them became cases. The exercise assumes complete follow-up, a consistent case definition and no one counted twice. It asks for attack proportions and the ratio between exposed and unexposed groups, then asks what the association can establish. No pathogen has been confirmed, and other shared foods or differing exposures may matter. The health teacher should coordinate immediate control and public-health communication rather than wait for arithmetic to prove a specific cause.