While you suspect the outbreak started at the food court, you need to confirm your suspicions. So, you decide to interview other students from campus who are not sick ("controls").
You're able to talk with 273 students who don't have any symptoms. You discover that most of these students did not eat at the food court during March 7-10 and instead chose to eat at one of the other cafeterias on campus. However, 100 of the students who are not sick did eat at the food court during this time.
In order to identify why some students got sick and others did not, you need to identify the source of the exposure. So, you ask the students about the foods they ate at the food court and also interview the staff who work in the food court.