“If you’re looking at potentially a problem, you want to understand how the people who are sick differ from those who are not sick and try to figure out, is there something the people affected used, something they did ... that made it more likely they fell ill,” said IDPH Director Dr. Ngozi Ezike. “It’s not enough just to look at the people affected; you also want to look at the people not affected. We’re trying to create the control group, if you will.”