Claimants seeking compensation for conditions that are the result of combat have a reduced evidentiary burden (sometimes called the "combat presumption"). Where a veteran "engaged in combat with the enemy in active service . . . the Secretary shall accept as sufficient proof of service-connection of any disease or injury alleged to have been incurred in or aggravated by such service satisfactory lay or other evidence of service incurrence of aggravation." 38 U.S.C. 1154(b); see also 38 C.F.R. § 3.304(d) ("Satisfactory lay or other evidence that an injury or disease was incurred or aggravated in combat will be accepted as sufficient proof of service connection if the evidence is consistent with the circumstances . . . of such service even though there is no official record of such incurrence or aggravation"). Section 1154(b) does not eliminate the need for medical nexus evidence; it merely reduces the burden of presenting evidence of incurrence or aggravation of an injury or disease incurred in or aggravated by combat service. Collette v. Brown, 82 F.3d 389, 392 (Fed. Cir. 1996).
Even when the combat presumption applies, a "veteran seeking compensation must still show the existence of a present disability and that there is a causal relationship between the present disability and the injury, disease, or aggravation of a preexisting injury or disease incurred during active duty." Shedden v. Principi, 381 F.3d 1163, 1167 (Fed. Cir. 2004); see also Boyer v. West, 11 Vet. App. 477, 478-79 (1998). Section 1154(b) does not eliminate the need for evidence of a medical nexus; it merely reduces, for veterans who have engaged in combat with the enemy, the burden of presenting evidence of incurrence or aggravation of an injury or disease in service. Collette v. Brown, 82 F.3d 389, 392 (Fed. Cir. 1996) ("Section 1154(b) does not create a statutory presumption that a combat veteran's alleged disease or injury is service-connected."); Jensen v. Brown, 19 F.3d 1413, 1416-17 (Fed. Cir. 1994); Clyburn v. West, 12 Vet. App. 296, 303 (1999).