In this image provided by the Centers for Disease Control and Prevention, a photomicrograph of a fresh stool sample, which had been prepared using a 10% formalin solution, and stained with modified acid-fast stain, reveals the presence of four Cyclospora cayetanensis oocysts in the field of view. Iowa and Nebraska health officials said Tuesday, July 30, 2013, that a prepackaged salad mix is the source of a cyclospora outbreak that sickened more than 178 people in both states. Cyclospora is a rare parasite that causes a lengthy gastrointestinal illness. (AP Photo/Centerd for Disease Control and Prevention)
State and federal investigators are trying to figure out the source of a stomach bug that has sickened 372 people in 15 states.
Iowa and Nebraska have linked some of their cases to pre-packaged salad mix.
But federal health officials have said the mix is not linked to all of the cases.
No cases have been reported in the state of Michigan.