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 that they retrieve a serum in exchange for his information about the Institute. From this point, the quest cannot be advanced until the player character makes their way to the Institute later in the game.

When in the Institute, find the FEV lab, an old locked area of Bioscience. There's a back entrance with a Master terminal in the main room of Bioscience. 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:
 * One 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 character's choice to cure Virgil, but Danse won't like it at all; this is due to what happened with his friend Cutler which resulted in a strong hate from Danse towards super mutants and other non-human creatures alike. 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.
 * One can also lie to Virgil through a sequence of three speech checks, (the difficulty of each one increases each time,) claiming that the serum wasn't there. 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 that the player character kill him and the quest will fail. If the player character has visited Swan's Pond and read the notes there, they can ask Virgil if he remembers Edgar Swann after the first speech check. This will lead to a medium speech check to convince him to end his life (bypassing the hard check).

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 the player character his lab for use, which will make all items therein available for looting.

Bugs

 * Virgil may become hostile while one is wearing a X-01 power armor suit or a power armor helmet with the targeting HUD 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.
 * Using the console to duplicate the experimental serum before completing the quest, will result in it being uncompletable. As the option to give the serum to Virgil will have its text removed. Clicking that option anyway will result in either nothing happening or activate the cutscenes with no dialogue.