This type of claims is real common where the disability is diabetes since so many service connected conditions can cause diabetes. Legal definition is “except as provided by section 3.300(c) disability which is proximately due to or the result of a service connected disease or injury shall be service connected. When service connection is thus established for a secondary condition, the secondary condition shall be considered part of the original condition. 38 CFR 3.310; Roper v. Nicholson, 20 Vet. App. 173, 181 (2006).
Secondary service connection can be established both where a service connected condition contributes to the creation of a new disability and where a service connected disability aggravates a non service disability. Mcqueen v. West, 13 Vet. App. 237, 241 (1999). It does not matter when after service a secondary disability manifests itself, as long as there is sufficient medical evid to establish that it is a result of a service connected condition. Dyess v. Derwinski, 1 Vet. App. 448, 454 (1991).