The Centers for Disease Control and Prevention (CDC) has launched an investigation into a multistate E. Coli outbreak after it sickened nearly 30 people in Michigan and Ohio.

The federal agency announced via its website on Wednesday that it will help investigate the outbreak of E. coli O157 after 29 people got sick because of it. At the time, nine of them got hospitalized due to the infection, while no deaths got reported.

The infections involving the initial 29 individuals occurred in late July. The ages of the people who caught the bacteria ranged from 6 to 91.

According to the CDC, the source of the outbreak was still unknown. It has since tried to find out if contaminated food caused the outbreak that struck Michigan and Ohio.

On Friday, the public health agency published an official investigation notice on the issue, and the updated figures showed that the outbreak has already sickened 37 people.

A specific food has not been confirmed as the source of the infections, but many of those who got sick from the bacteria reported eating sandwiches with romaine lettuce at Wendy’s restaurants in Michigan, Ohio and Pennsylvania.

After getting notified and as a safety precaution, Wendy’s promptly removed all romaine lettuce from its sandwiches served in restaurants in the affection region.

“Investigators are working to confirm whether romaine lettuce is the source of this outbreak, and whether romaine lettuce used in Wendy’s sandwiches was served or sold at other businesses. Wendy’s is fully cooperating with the investigation,” the CDC indicated.

The agency clarified that it is not discouraging the public from eating at Wendy’s restaurants, or for people to stop eating romaine lettuce while it is still investigating the matter.

But for those who developed symptoms like diarrhea, fever, vomiting and dehydration after eating a certain food item within the affected region, the CDC said it’s crucial to seek help from a healthcare provider right away.

While the symptoms of Shiga toxin-producing coli vary from person to person, the hallmark symptoms remain the same in most cases, including bloody diarrhea, fever that is not very high (101˚F/38.5˚C), stomach cramps and vomiting.

Infections caused by E. coli are mostly very mild, and patients get better within 5 to 7 days. However, some cases experience severe or even life-threatening illness that requires immediate medical attention.