JC doesn't need Grey Death treatment. There's actually indications that Grey Death is connected to whatever genes or technology allow the Dentons to tolerate nanotech augmentation. For example, when you find Paul in the MJ12 lab a datacube nearby says Paul tests positive for Grey Death but curiously does not develop symptoms.

I could swear someone actually tells JC he doesn't need treatment for Grey Death because "...you are Grey Death". That might be cut dialogue that the GMDX mod restores in the game.

JC doesn't need Grey Death treatment. There's actually indications that Grey Death isn't just a disease but also a way for certain people to tolerate nanotech augmentations. For example, when you find Paul in the MJ12 lab a datacube nearby says Paul tests positive for Grey Death but curiously does not develop symptoms. There's some other hints that truly curing Grey Death (Ambrosia is not a cure but a temporary treatment) would also provide the key to letting anyone use nano augmentations.

I could swear someone actually tells JC he doesn't need treatment for Grey Death because "...you could say you are Grey Death". That might be cut dialogue that the GMDX mod restores in the game.