I've got a pet house cat. She stays indoors
My daughter has a cat.  We just had to take her for the feline leukemia shot.
I'm vaguely familiar with feline leukemia virus. I know it was called  FeLV when I worked at a vet clinic. 
It's terrible.  I had an outside cat when I was younger that got it.  I had to have him put to sleep.  He couldn't breathe and his lungs filled with fluid.
That's too bad..I'm sorry for your loss! I didn't know it had such an impact on their immune system. 
I don't really know what it does, but I do know it's fatal.  So if we are going to have a cat, it has to have that vaccine.
That's a good idea, I didn't know there was a vaccine, but that would definitely help boost immunity. I bet keeping the cat indoors would help, too, since FeLV can be transmitted from unvaccinated stray cats.