POSSIBLE ALLERGY TO ANTIBIOTICS
When I first got Roo, he had an upper respiratory infection.  After taking antibiotics for awhile, he got worse instead of better, and when we switched antibiotics, there was still no improvement.  I started preparing to face the fact that he was a sickly sort of cat and probably wouldn't live very long.   As it turned out, the antibiotics were making him sick.  I was never sure whether he was allergic to them or just overly sensitive.  After I stopped giving them, he perked right up, had remarkably few problems for most of the rest of his life, and lived longer than any other cat I've ever had.
PROBABLE INTESTINAL CANCER / PROBABLE HYPERTENSION
Early in 1999, Roo had to have an abscessed tooth removed.  While doing the dental work, the vet noticed a small lump under his tongue.  A biopsy determined it was pre-cancerous.  We kept an eye on his mouth after that, but didn't notice anything. 
In June of 1999 he started having chronic diarrhea, which we treated several times with no success.  In July, he got quite ill and had to be hospitalized for several days.  Lab work revealed nothing at all, but the symptoms suggested intestinal cancer.  It didn't seem right to subject him to a bunch of stressful, invasive tests to diagnose something untreatable.  We started him on steroids and gave him what time he had.
During the first couple of months of summer, he abruptly lost his vision, also.  This was before most vets had the ability to measure blood pressure in animals, but a common cause of sudden vision loss in cats is detached retinas caused by hypertension. I suspect that's what happened in Roo's case.  

Roo did well on the steroids for much longer than we expected he would, and had no problems finding his way around the house - including up and down the stairs - in spite of being blind.  He also ate like a pig, courtesy of the medication.  One day though, he started going downhill fast and it was obvious he couldn't fight it anymore.  It was time to say goodbye - after almost 14 years.   He was around for so long it still seems strange that he's gone now.