@urtoast
...Perhaps... though it wasn’t explained what caused her to suddenly start physically disappearing in the first place or why. Kuuko presents a theory, but even that isn’t explained.
Had it just been a really bad fever (with delirium, etc), I could see the illness effecting her enough to cause a believable temporary loss of powers that would reappear once she was fully recovered. But it still wouldn’t explain her physically disappearing into another plane of existence, let alone one that is not the realm of the Ayakashi (Kuuko’s theory). It just wasn’t very well explained.