A form of the E. coli bacteria lurking in a popular caterer's potato salad caused more than 4,000 people to fall ill, health officials said. And in Georgia, a virulent form of E. coli sickened at least six children.

The Illinois case was the largest documented outbreak of the bacteria strain called enterotoxigenic Escherichia coli in the United States, Cook County Health Department officials said Monday.The bacteria, known as ETEC and nicknamed "traveler's diarrhea," was identified in stool samples from three victims, said Dr. Stephanie Smith, the department's director of communicable disease.

Officials from the federal Centers for Disease Control and Prevention were conducting tests to isolate the bacteria in the food or water at Iwan's Deli and Catering in Orland Park, a Chicago suburb.

Although common in developing countries, the CDC in Atlanta has tracked just 14 ETEC outbreaks in the U.S. in the past 23 years, Smith said.