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5.4. Medical Conditions Aggravated by "Service"
VA will compensate claimants for medical conditions that existed at the time of entry into service that were made worse or "aggravated" by service. The essence of a claim for benefits based on a theory of aggravation is that a claimant's service caused a worsening of a preexisting condition. See Wagner v. Principi, 370 F.3d 1089, 1096 (Fed. Cir. 2004) ("[I]f a preexisting disorder is noted upon entry into service, the veteran cannot bring a claim for service connection for that disorder, but the veteran may bring a claim for service connected aggravation of that disorder.").
An appellant may obtain service connection for aggravation of a preexisting condition under 38 U.S.C. section 1153. In such a case, "the burden falls on the veteran to establish aggravation." Wagner, 370 F.3d at 1096. If the veteran succeeds in showing aggravation, "the burden shifts to the government to show . . . that the increase in disability is due to the natural progress of the disease." Id. Where there has been an increase in disability during service, the proof that the increase was due to the natural progress of the disease must also be by clear and unmistakable evidence. 38 C.F.R. § 3.306(b). Therefore, the first task for the Board in evaluating a presumption of aggravation claim is to find whether the appellant has shown an increase in disability during service. If the Board finds aggravation, the second task is for the Board to consider whether the increased disability is due to the natural progression of the disease. See Wagner, 370 F.3d at 1096.
 Also see the discussion of the "presumption of soundness" as it applies to determining if a medical condition pre-existed service.