According to Illinois Pattern Instruction (IPI) 30.03, the "aggravation of any pre-existing ailment or condition" is a separable element of compensable damages under Illinois law. However, as the comment to IPI 30.03 notes, the First District disagrees about this, as evidenced by Smith v. City of Evanston, 260 Ill.App.3d 925, 631 N.E.2d 1269, 197 Ill.Dec. 810 (1st Dist. 1994).
In Luye v. Schopper, No. 1-02-0610 (1st Dist. 2004), the First District reconsidered the Smith holding and reaffirmed it, stating that IPI 30.03 misstates the law and should not be used:
In Smith, the appellate court determined aggravation of a preexisting condition did not constitute a separate element of damages because it would overcompensate plaintiffs due to the overlap of aggravation of a preexisting injury with awards for other elements of damages. Smith, 260 Ill. App. 3d at 935-36. The Smith court held that although the aggravation of a preexisting injury should be taken into account in awarding damages, it was error for the trial court to give the jury an instruction and itemized verdict form with aggravation of a preexisting condition as a separate element of damages. Smith, 260 Ill. App. 3d at 935.
IPI 30.03 remains viable in some other districts; be sure to conduct your own research before using this instruction (or objecting to its use).