The Illinois Department of Health is investigating a salmonella outbreak that has sickened almost 100 people who have reported eating at Subway sandwich restaurants in 28 counties around the state.

Investigators had confirmed 97 cases of Salmonella Hvittingfoss infection as of Monday morning, according to the health department’s Web site.

Representatives from the Illinois Department of Health and Subway were not immediately available for comment.

Subway is owned by Doctor’s Associates Inc. and has more than 28,500 locations in 86 countries.