38 CFR § 3.306 - Aggravation of preservice disability.
(a) General. A preexisting injury or disease will be considered to have been aggravated by active military, naval, or air service, where there is an increase in disability during such service, unless there is a specific finding that the increase in disability is due to the natural progress of the disease.
(b) Wartime service; peacetime service after December 31, 1946. Clear and unmistakable evidence (obvious or manifest) is required to rebut the presumption of aggravation where the preservice disability underwent an increase in severity during service. This includes medical facts and principles which may be considered to determine whether the increase is due to the natural progress of the condition. Aggravation may not be conceded where the disability underwent no increase in severity during service on the basis of all the evidence of record pertaining to the manifestations of the disability prior to, during and subsequent to service.
(1) The usual effects of medical and surgical treatment in service, having the effect of ameliorating disease or other conditions incurred before enlistment, including postoperative scars, absent or poorly functioning parts or organs, will not be considered service connected unless the disease or injury is otherwise aggravated by service.
(2) Due regard will be given the places, types, and circumstances of service and particular consideration will be accorded combat duty and other hardships of service. The development of symptomatic manifestations of a preexisting disease or injury during or proximately following action with the enemy or following a status as a prisoner of war will establish aggravation of a disability.
(c) Peacetime service prior to December 7, 1941. The specific finding requirement that an increase in disability is due to the natural progress of the condition will be met when the available evidence of a nature generally acceptable as competent shows that the increase in severity of a disease or injury or acceleration in progress was that normally to be expected by reason of the inherent character of the condition, aside from any extraneous or contributing cause or influence peculiar to military service. Consideration will be given to the circumstances, conditions, and hardships of service.