Last year, one of my cats at the shelter tested positive for FIV. That's Feline Immunodeficiency Virus. It's similar to HIV in humans, but is not cross contagious. In other words, you won't get it from your cat. And it's not even terribly contagious between cats. They can, therefore, live long and prosper... but you have to take precautions.
It usually is transferred through biting or other forms of body fluid exchanges, such as mutual grooming and sharing food bowls, though the "injection" method is the most risky... that is, injecting each other with the virus through biting and scratching. Thus, it's best to keep such cats as single pets.
However, since shelters are always on the lookout for a safe place to send pets no one wants, word gets out fast if someone has room for an FIV cat. And so it came to be that I now have 4 cats here with immune diseases. They live together, but do not mingle with the other cats.
They seem healthy and are getting along fine. But they are not adoptable now, and so they will stay with me permanently.
If you have such a cat, you have two choices: euthanize the cat, or keep it as a single pet, indoors only. OK, you have a third choice, but it's the hardest one of all: Try to find someone who is willing to take on a cat with a health risk. It's not easy, and not likely, either. Be sure to donate to your local shelter so they can continue to care for the unwanted pets with no place to go.
To learn more about this disease, you can find some information at my main web site, The Problem Cat.
There are several feline diseases about which little is known, and include FIV, FeLV (feline leukemia) and FIP (feline infectious peritonitis). It's an area where not many research dollars are spent.