Most states require that a health certificate, Certificate of Veterinary Inspection (CVI), accompany horses entering from another state.
A CVI is attained from an accredited veterinarian who examines your horse.  The purpose of the certificate is to attest that the horse shows no obvious signs of disease on the day the examination was made.  This certificate is signed by the veterinarian and is good for 30 days.
The state of Illinois also requires an entry permit number which is issued by the Illinois Department of Agriculture and is good for the life of the CVI, which is 30 days.
THERE IS NO CHARGE FOR OBTAINING AN ENTRY PERMIT NUMBER FROM THE ILLINOIS DEPARTMENT OF AGRICULTURE.
Entry permit numbers are available by calling the Illinois Department of Agriculture, Monday through Friday from 8:00 AM to 4:30 PM, at 217/782-4944.  Permits may also be obtained online at:  http://www.agr.state.il.us/AnimalHW/animalregistry/login.php