Virgil's letter is a paper note in Fallout 4.
Location
Given by Brian Virgil to the Sole Survivor if he is cured.
Transcript
Doctor,
Our mutual friend informed me of your concern for my health. With the aid of <Alias.PronounPosObj=Player>, I was able to secure an antidote for my condition, and am pleased to report that I have made a full recovery.
Although our circumstances have changed, I hope we might someday be able to collaborate on our research once again. Until then, my regards.
Sincerely,
V.