The patient is the one the possession is "apparent" to.

The way they worded it indicates the condition (a delusion) is genuine, in that trivial efforts to assist the patient do not discard the "possession".

Like, I feel I've been in a haunted place before, but collected my thoughts and realized it was an old building with a draft and old framing. It still creeped me out for the rest of the night but I reassured myself that it was my social conditioning of what "creepy" was that was making me feel that way. If I couldn't shake the fear, that would make my perception more apparent.