Big U.S. restaurant chains don’t get linked to foodborne illness outbreaks often. But the number of meals they serve causes a lot of concern when contamination of some kind sickens customers.

“Early information has shown lettuce as a common product that regularly comes up during the investigation,” said the Michigan health department.

Thousands of sick people didn’t eat at Taco Bell, and investigators are racing to discover where else the contaminated lettuce was sent.