Whenever any disability from which any employee is suffering following the contraction of a compensable occupational disease is due in part to such occupational disease and in part to a preexisting disease or infirmity, the Board shall determine the proportion of such disability which is reasonably attributable to the occupational disease and the proportion which is reasonably attributable to the preexisting disease or infirmity and such employee shall be entitled to compensation only for that proportion of the disability which is reasonably attributable solely to the occupational disease.