If it appears that a permanent disability of a covered employee following an accidental personal injury or occupational disease is due partly to the accidental personal injury or occupational disease and partly to a preexisting disease or infirmity, the Commission shall determine: the proportion of the disability that is reasonably attributable to the accidental personal injury or occupational disease; and the proportion of the disability that is reasonably attributable to the preexisting disease or infirmity. The covered employee: is entitled to compensation for the portion of the disability of the covered employee that is reasonably attributable solely to the accidental personal injury or occupational disease; and is not entitled to compensation for the portion of the disability that is reasonably attributable to the preexisting disease or infirmity.