Yesterday, we lost our youngest cat, Jack Jr. He was a purebred Russian Blue, and the kindest, most loving cat I've ever had the joy of knowing. He would have been five years old next month.
We found out today that he was born with a congenital kidney defect. Cats born with the particular defect he had will appear to be healthy, and will not present any symptoms whatsoever until they've lost over 75% of their kidney function and their bone marrow stops producing new red blood cells. Unfortunately in his case, there's nothing we could have done to save him even if we'd known.
Please, if you have a purebred pet of any breed, have them screened. Purebreds are much more likely than mixed breeds to have health conditions.