U.S. officials have traced an explosive diarrhea-causing parasite outbreak in five states to shredded lettuce served at the popular fast-food chain

Michigan, which is where much of the outbreak is centered, has reported over 4,300 cases of cyclosporiasis, which can cause diarrhea and a low grade fever.

Nearly 7,000 cases nationwide are confirmed or under investigation, according to the CDC. Michigan has accounted for the bulk of cases so far.