Virgil's Cure is a quest that can be obtained within the Commonwealth, in Fallout 4.
In return for supplying the schematics for the molecular beam emitter, Brian Virgil asks the Sole Survivor for a single favor: Bring back the FEV cure tailored to his DNA. He does not want to live out the rest of his years as a mutated monstrosity. Fortunately, since Volkert was always envious of Virgil's work, the lab will still be sealed up as he goes through Virgil's work.
- The cure is located within the FEV lab, in the room with the test chambers, inside a machine. Simply travel to the Institute and retrieve it, fighting through the defenses. Once returned to Virgil, he will inject himself and become human after 72 hours pass. This also allows the player to complete A Loose End without killing Virgil, by convincing Kells that the scientist is trapped in the Glowing Sea and monitoring him will suffice.