The other day I spent a few hours in dev mode doing various tests to see what would spread metalhorror implantation- things like surgery, tending, feeding meals, sharing a barracks, etc.
I suspect but would likely need someone to check the source to confirm that attacks from entities such as Fingerspikes, Noctols, and Ghouls do not actually spawn new metalhorror implantations- I believe that a separate event (visible in dev mode as MetalHorrorImplantation) fires and then checks back through what has happened to pawns and if one has a qualifying event, they will be chosen as the first victim of a new metalhorror implantation.
If you wish to test in dev mode, I suggest doing the following:
- Ensure that you are shown hidden hediffs
- Try the MetalHorrorImplantation event with a fresh batch of colonists. It should fail silently with none gaining the corresponding hediff.
- Spawn a fingerspike and allow it to wound a colonist. Colonist probably won't receive the metalhorror implantation hediff.
- Try the MetalHorrorImplantation event again. The attacked colonist should receive the metalhorror implantation hediff listing a fingerspike attack as the source.
I am pretty sure wanderers who join, and creepjoiners who have joined your colony and don't have the metalhorror downside are valid recipients with the implanted before arrival at the colony source.