The Saluki Express hit the wall Tuesday.
One of the lines' buses crashed into the wall of a shopping center in downstate Carbondale, injuring four people after the driver had some sort of "medical episode," authorities said.
The Southern of Carbondale reported that the driver and three passengers were taken to a hospital. The extent of their injuries is unknown.
Saluki operates between the University Place Shopping Center and other areas around Carbondale while Southern Illinois University is in session.
The bus crashed into a wall of a movie theater. Officials there say a structural engineer will be contacted to examine the damage.