38 CFR § 3.322 - Rating of disabilities aggravated by service.
(a) Aggravation of preservice disability. In cases involving aggravation by active service, the rating will reflect only the degree of disability over and above the degree of disability existing at the time of entrance into active service, whether the particular condition was noted at the time of entrance into active service, or whether it is determined upon the evidence of record to have existed at that time. It is necessary to deduct from the present evaluation the degree, if ascertainable, of the disability existing at the time of entrance into active service, in terms of the rating schedule except that if the disability is total (100 percent) no deduction will be made. If the degree of disability at the time of entrance into service is not ascertainable in terms of the schedule, no deduction will be made.
(b) Aggravation of service-connected disability. Where a disease or injury incurred in peacetime service is aggravated during service in a period of war, or conversely, where a disease or injury incurred in service during a period of war is aggravated during peacetime service, the entire disability flowing from the disease or injury will be service connected based on the war service.