CHICAGO — The Subway food service chain has been linked to an outbreak of foodborne illnesses due to Salmonella Hvittingfoss in Illinois, according to the Illinois Department of Public Health (I.D.P.H.). As of June 8, 60 people ranging in age from 2 years old to 88 have been sickened.

The I.D.P.H. said consumers involved in the outbreak reported eating at Subway locations in 18 Illinois counties. No specific food source has been identified, but the I.D.P.H. is working with the Food and Drug Administration, Centers for Disease Control and Prevention as well as Subway to identify the source of the illnesses.

Salmonella Hvittingfoss is an uncommon strain. The I.D.P.H. said only one or two cases of foodborne illness associated with the strain are typically seen in the state per year.