[ training ] . " Id . at 5 . Generally , service connection may be granted for a disability resulting from disease or injury incurred coincident with or aggravated by service . 38 U . S . C . A . § 1131 ; 38 C . F . R . § 3 . 303 ( a ) . Service connection requires evidence satisfying three criteria : ( 1 ) the existence of a present disability ; ( 2 ) in - service incurrence or aggravation of a disease or injury ; and ( 3 ) a causal relationship ( " nexus " ) between the present disability and the disease or injury incurred or aggravated during service . Shedden v . Principi , 381 F .