Yes, this is the feline variant of the human HIV. FIV attacks the immune system of cats, much like the human immunodeficiency virus (HIV) attacks the immune system of human beings. It is the third virus in the trio of lethal, incurable sleeper-diseases. For the record, FIV is not transmittable to humans.

How does it spread?

The virus is transmitted via mucosal surfaces such as those in the mouth, rectum, and vagina. The most common route of infection is via deep bite wounds. This is why it is typically a disease we see in stray cats and outdoor cats.
There is no vaccine at this present time which can prevent FIV!

What does it do?

It disables the immune system, leaving the cat vulnerable to infection. Even the silliest infection can mean great discomfort since the cat has no immune system to ward herself against the infection. Eventually a simple respiratory infection, for instance, can become a death-cause, in this case.
This does not mean, however, that you must immediately euthanize a cat with FIV. They are perfectly capable of living a happy live, if you take certain precautions. Yes, you will spend more time at the vet, since you have to stay on top of health issues, to make sure you catch it in time. And you have to keep the cat inside so it cannot infect others. But other than that they can live for many years, just like any other pet. And if you have several cats, you may want to separate them. This is however not necessary if they do not fight.

How do I tell?

The disease occurs in three stages:
First is the Acute Stage (1-2 months after transmission) in which fever, depression, and “generalized lymphadenopathy” are observed.

Second is the Subclinical Stage (4 weeks to X months after transmission), in which symptoms of the disease decrease or disappear; however, all cats remain viremic for life.

Third is the Chronic Stage, in which cats eventually succumb to chronic infections due to suppressed immune system function. Cats may incur stomatitis, odontoclasia, periodontitis, gingivitis, rhinitis, conjunctivitis, pneumonitis, enteritis, and dermatitis in the later stages of infection.
If your cat develops a serious infection, for instance a serious infection of the gums or mouth in general (stomatitis), you should have the vet check it out anycase. They can indicate FIV, but more often then not they are unrelated infections which can be treated easily. It is really easy to diagnose FIV. Contrary to FIP, you can administer a simple blood-test which will tell you yes or no. If it does say your cat is negative, it is suggested to repeat the blood-test after 2 months just to make sure.
FIV, as said before, is not the end of the world. There are even shelters who will allow you to adopt an FIV-positive cat. And if it is the only cat you have, then it really does not matter. They can be as fun and loving as normal cats. The only drawback is the more frequent ratio of vet-visits. But if you are just a tad unlucky with your kitty (you have for instance a boisterous male who insists on fighting everyone he meets just to show off how macho he is), you will probably end up even more at the vet’s office.

Hopefully I was able to demystify FIV a bit here, and take away the fear that is commonly associated with aids. If you are looking to adopt a kitty, take a second to consider an FIV-positive cat ;)