Virgil's Cure

Virgil's Cure is a miscellaneous quest in Fallout 4.

Detailed walkthrough
Virgil's Cure automatically starts when talking to Brian Virgil in the main story quest "The Glowing Sea". In the conversation the Sole Survivor first has with Virgil, he requests serum in exchange for his information about The Institute. From this point, the quest cannot be advanced until the player makes their way to the Institute later in the game.

When in The Institute, find the FEV lab, an old locked area of the Bioscience lab. There's a back entrance with a master lock in the main room of the Bioscience lab. This leads directly to the room the serum is in, and also the main entrance to the FEV lab, found at the end of some seemingly abandoned storage halls and rooms. Once in the FEV lab, travel through it to the end room, where the serum is located. The lab will be defended by several turrets and also an assaultron, but is otherwise uninhabited. The serum is in the final room in a lab machine on top of a desk.

Bring the serum back to Virgil. The player character has two options:
 * You can simply give Virgil the serum, and certain companions will react differently to this choice. For example, John Hancock, Piper, Nick Valentine, Codsworth and Curie like the player's choice to cure Virgil, but Danse won't like it at all; this is due to his teachings by the Brotherhood. Strong also won't like it as he prefers being a super mutant and is angry with the doctor's ambitions to cure the others.
 * You can also lie to Virgil through a sequence of three speech checks, claiming that the serum wasn't there. Checks difficulty increases with each one. After the first two checks the player character still has the option to give the serum to Virgil through breaking and re-initiating dialog with him. After the third speech check Virgil requests the player character to kill him and the quest will fail.

After giving the serum to Virgil, wait three days and return to find he has transformed back into a human. Speak with him again and he'll offer you his lab for use, which will make all items therein available for looting.

Bugs

 * Virgil may become hostile while you are wearing a X-01 power armor suit or a power armor helmet with the targeting mod installed.
 * This miscellaneous quest may not be given but if you return the cure without having the quest anyway, it may continue even though a marker was not given.