People became ill after eating potato salad prepared by Iwan's Deli and Catering. The firm had prepared food for more than 530 parties over the weekend of June 6. Between June 5 and 7, a delicatessen catered 539 events that were attended by over 16000 people. In addition, it had offered restaurant seating and take out. After receiving reports of illness from patrons eating foods served through all three service routes, the Cook County Health Department investigated and confirmed that the illnesses were caused by E. coli O6:H16. Multiple food items (i.e., potato salad, macaroni salad, egg salad, and watermelon) were associated with gastroenteritis. The delicatessen had inadequate hand-washing supplies, inadequate protection against back siphonage of wastewater in the potable water system, a poorly draining kitchen sink, and improper food storage and transportation practices.