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.
Vaccine
A vaccine is a biological preparation that provides active acquired immunity to a particular disease. A vaccine typically contains an agent that resembles a disease-causing microorganism and is often made from weakened or killed forms of the microbe, its toxins, or one of its surface proteins. The agent stimulates the body's immune system to recognize the agent as a threat, destroy it, and to further recognize and destroy any of the microorganisms associated with that agent that it may encounter in the future. Vaccines can be prophylactic (
went into effect on April 30, 2011, and survived a court challenge from pet stores.
Programs.
Programs Cat Sanctuary.
The cat sanctuary, located on six acres (2.4 hectares) of suburban farmland, has been described as "Club Med for cats". There are two main buildings and twelve smaller ones, including a kitten house and two houses for cats with the Feline immunodeficiency virus. Another area is provided for cats with the Feline Leukemia Virus. "Enclosed decks on the main buildings provide indoor cats