New Delhi, July 17 -- LANSING - The watery diarrhea started without warning. For Michigan residents who sought emergency care in recent weeks, the culprit turned out to be a microscopic parasite called Cyclospora cayetanensis, lurking, investigators now believe, in shredded iceberg lettuce served at Taco Bell restaurants in five Midwestern and Appalachian states.

Federal investigators have determined that a single unnamed Mexican supplier is the source of contamination driving the nation's largest cyclospora outbreak in recent memory. Taco Bell confirmed Thursday it has voluntarily removed shredded iceberg lettuce from locations in Indiana, Kentucky, Michigan, Ohio, and West Virginia. No formal recall has been ordered by the Food and Dru...