Eight of 10 family practice residents who had a potluck 4 days ago now have diarrhea with abdominal cramps, general malaise, and fever ranging from 37.5° to 38.7°C. Stools from 3 residents are blood tinged. Laboratory studies revealed the causal agent was a microaerophilic gram-negative, curved rod with polar flagella often in pairs to give a "seagull" appearance. It grew on special media at 42°C. The original contamination probably was found in