38 U.S.C. § 1132

For the purposes of section 1131 of this title, every person employed in the active military, naval, air, or space service for six months or more 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 evidence or medical judgment is such as to warrant a finding that the disease or injury existed before acceptance and enrollment.