40 ILCS 5/15-151
Disability benefits shall begin to accrue upon the termination of the payment of salary or sick leave benefits or the 61st day after the occurrence of the disability, whichever is later. However, no benefits shall be payable covering a period of more than 30 days prior to the receipt of a written application unless the board finds good cause for the delay in filing the application. The recurrence within 30 days of a former disability shall be considered a continuation of the disability. If a disabled participant returns to his or her assigned position and within 30 days again becomes disabled from the same cause, the previous period of disability shall be considered in determining the date benefits may begin, and the amount of the benefit shall be based upon the basic compensation on the date the participant first became disabled from this cause.
40 ILCS 5/15-151