SEVERE EYE INFECTION / PERFORATION
When Chris found her, Penelope was just a little ball of grey fur covered in cactus spines and she had a horrific eye infection, which had sealed one eye shut and the other almost shut.  Medical care was obviously an urgent necessity, so after we picked the cactus out of her, I dropped her at the vet's on the way to work.  The news from the vet was that her left eye had been perforated, either by the severe eye infection or by something she'd run into.  The right eye was infected but in good shape otherwise, and she had an upper respiratory infection and an ear infection.
There was no saving the left eye and she obviously had no vision in it anyway.  Before we could schedule surgery to remove it, we had to treat the infection in the other eye and the upper respiratory infection.  Once those problems were resolved, she made it through surgery just fine.  She got along as well as any cat and didn't seem to miss the eye in the least. 
FELINE LEUKEMIA (FeLV)
All Penelope's infections worried us, and also, she came from the same group of cats as Arivaca had, and Arivaca had been born with Feline Leukemia.  We had Penelope tested the day we found her and the results were negative, but we didn't trust them entirely, as she was awfully young at the time.  We grew even more suspicious when she had quite a bit of trouble getting rid of the upper respiratory infection.  Unfortunately, but not surprisingly, two subsequent FeLV tests were positive.  We think most of that group of cats is probably infected and they live just long enough to produce a litter or two of infected kittens.
Penelope was on alpha interferon for approximately the last 15 months of her life.  Interferon has been used with some success in FeLV positive cats; it's an immune-system booster, so it can help them fight off various infections.  It is in liquid form and is given orally, every day for a week, then off for a week, alternating weeks.  We were pleased to discover that it is quite moderately priced.  She initially took the interferon for a little over three months to treat a chronic upper respiratory infection, and it worked like a charm.  We kept her on it in the hope it might extend her life.  Whether it did or not, we don't know, but she lived longer than Arivaca did.

We knew she wouldn't have a normal life expectancy even with the interferon, and that was hard to take.  All we could do is make sure her life was blissful while we had her, and it certainly seemed to be.
MEDIASTINAL LYMPHOMA
Penelope's experience with lymphoma was like a horrid replay of Arivaca's.  Like Arivaca, she didn't show any noticeable symptoms until the last day of her life, when we noticed that her breathing was labored.  As soon as I saw that, I knew it was the end.  A trip to the vet confirmed our suspicions, and there was nothing to do at that point but put her down.

On the positive side, I don't think she ever felt bad until that last day.  It is never easy losing them that young, though.