The U.S. Centers for Disease Control and Prevention has found that an outbreak of E. coli bacteria in four Midwestern states from an unknown source has affected 47 more people, bringing the total to 84.

As of Wednesday, 38 people were hospitalized. Although many strains of E. coli bacteria are found in the intestines of healthy humans and animals and are harmless, some species can cause severe intestinal cramps, bloody diarrhea and vomiting.

The CDC said that of the 62 people surveyed, 52 ate at Wendy’s in the week prior to the onset of illness.

The burger chain confirmed to Reuters in an email that it is “cooperating fully with health authorities in their ongoing investigations into the local E. coli outbreak.”

The number of new cases is up from last Friday, when US centers launched an investigation after 37 people were infected in Michigan, Ohio, Indiana and Pennsylvania.

The centers said many of them ate romaine lettuce sandwiches at Wendy’s before they got sick.

However, the CDC did not advise eating at Wendy’s and did not ask people to avoid romaine lettuce, as the exact cause of the outbreak has not yet been determined.

As a precautionary measure, the company that owns the restaurant chain has confirmed salad substitutions in sandwiches in some of the affected areas.

And American centers have found that the actual number of cases is likely to be higher than reported, and the outbreak may not be limited to the states where cases appeared.