38 CFR § 3.303 - Principles relating to service connection.
(a) General. Service connection connotes many factors but basically it means that the facts, shown by evidence, establish that a particular injury or disease resulting in disability was incurred coincident with service in the Armed Forces, or if preexisting such service, was aggravated therein. This may be accomplished by affirmatively showing inception or aggravation during service or through the application of statutory presumptions. Each disabling condition shown by a veteran's service records, or for which he seeks a service connection must be considered on the basis of the places, types and circumstances of his service as shown by service records, the official history of each organization in which he served, his medical records and all pertinent medical and lay evidence. Determinations as to service connection will be based on review of the entire evidence of record, with due consideration to the policy of the Department of Veterans Affairs to administer the law under a broad and liberal interpretation consistent with the facts in each individual case.
(b) Chronicity and continuity. With chronic disease shown as such in service (or within the presumptive period under § 3.307) so as to permit a finding of service connection, subsequent manifestations of the same chronic disease at any later date, however remote, are service connected, unless clearly attributable to intercurrent causes. This rule does not mean that any manifestation of joint pain, any abnormality of heart action or heart sounds, any urinary findings of casts, or any cough, in service will permit service connection of arthritis, disease of the heart, nephritis, or pulmonary disease, first shown as a clearcut clinical entity, at some later date. For the showing of chronic disease in service there is required a combination of manifestations sufficient to identify the disease entity, and sufficient observation to establish chronicity at the time, as distinguished from merely isolated findings or a diagnosis including the word “Chronic.” When the disease identity is established (leprosy, tuberculosis, multiple sclerosis, etc.), there is no requirement of evidentiary showing of continuity. Continuity of symptomatology is required only where the condition noted during service (or in the presumptive period) is not, in fact, shown to be chronic or where the diagnosis of chronicity may be legitimately questioned. When the fact of chronicity in service is not adequately supported, then a showing of continuity after discharge is required to support the claim.
(c) Preservice disabilities noted in service. There are medical principles so universally recognized as to constitute fact (clear and unmistakable proof), and when in accordance with these principles existence of a disability prior to service is established, no additional or confirmatory evidence is necessary. Consequently with notation or discovery during service of such residual conditions (scars; fibrosis of the lungs; atrophies following disease of the central or peripheral nervous system; healed fractures; absent, displaced or resected parts of organs; supernumerary parts; congenital malformations or hemorrhoidal tags or tabs, etc.) with no evidence of the pertinent antecedent active disease or injury during service the conclusion must be that they preexisted service. Similarly, manifestation of lesions or symptoms of chronic disease from date of enlistment, or so close thereto that the disease could not have originated in so short a period will establish preservice existence thereof. Conditions of an infectious nature are to be considered with regard to the circumstances of the infection and if manifested in less than the respective incubation periods after reporting for duty, they will be held to have preexisted service. In the field of mental disorders, personality disorders which are characterized by developmental defects or pathological trends in the personality structure manifested by a lifelong pattern of action or behavior, chronic psychoneurosis of long duration or other psychiatric symptomatology shown to have existed prior to service with the same manifestations during service, which were the basis of the service diagnosis, will be accepted as showing preservice origin. Congenital or developmental defects, refractive error of the eye, personality disorders and mental deficiency as such are not diseases or injuries within the meaning of applicable legislation.
(d) Postservice initial diagnosis of disease. Service connection may be granted for any disease diagnosed after discharge, when all the evidence, including that pertinent to service, establishes that the disease was incurred in service. Presumptive periods are not intended to limit service connection to diseases so diagnosed when the evidence warrants direct service connection. The presumptive provisions of the statute and Department of Veterans Affairs regulations implementing them are intended as liberalizations applicable when the evidence would not warrant service connection without their aid.