Staff at an Illinois prison were left needing medical treatment Wednesday after becoming sick while responding to inmates who were apparently under the influence of an “unknown substance,” a spokesperson said. Eighteen workers at John A. Graham Correctional Center in Hillsboro were rushed to area hospitals, while an undisclosed number of prisoners were treated at the facility’s health-care unit. The staff became sick after one worker responded to a “medical incident involving individuals in custody who appeared to be under the influence of an unknown substance,” state Department of Corrections spokesperson Naomi Puzzello said in a statement. “All staff members who may have potentially encountered the unknown substance were also sent to a local hospital, as a precaution. All staff are stable currently and many have already been discharged,” Puzzello added, confirming that an Illinois State Police hazardous materials team is investigating the incident.