38 U.S.C. § 1111 : US Code - Section 1111: Presumption of sound condition

      For the purposes of section 1110 of this title, every veteran
    shall be taken to have been in sound condition when examined,
    accepted, and enrolled for service, except as to defects,
    infirmities, or disorders noted at the time of the examination,
    acceptance, and enrollment, or where clear and unmistakable
    evidence demonstrates that the injury or disease existed before
    acceptance and enrollment and was not aggravated by such service.