One evening in November 1999, Bobby turned up looking very ill - so ill that Graeme took him right to the vet.  After an examination, the vet somehow came up with a diagnosis of diabetes.  How, we don't know, and it was obviously incorrect.
After the visit to the vet, Bobby continued to look bad and was displaying some odd neurological symptoms.  Graeme tried another vet, who did a full workup but was unable to arrive at a definitive diagnosis.  It initially looked like there was some displacement of one of the cervical (neck) vertebrae and for a while, the theory was that he'd somehow broken his neck.  That turned out not to be the case.
Some days were better than others, but neither antibiotics nor steroids nor Graeme's nursing care really seemed to help and no one was sure what was going on.  The neurological symptoms continued, then worsened, and ultimately, Bobby died.
The second vet asked to have an autopsy done since Bobby's case remained undiagnosed, and the autopsy finally provided the answer: he had died of a large brain abscess.  How he got the brain abscess, no one knows.   It's possible the whole problem could have started with a tiny puncture to the back of his neck that resulted in an abscess that never broke open (explaining the displaced cervical vertebrae) and the infection spread from there.

We will never know the whole story, but it was the last thing anyone expected to happen to an otherwise healthy, vigorous, five year old cat.  I have to wonder now if Orion didn't have something similar going on, though in his case, the infection was much lower down the spinal column and we were lucky enough it didn't spread.