Feline Immunodeficiency Virus (FIV) is a Group IV member of Family Retroviridae and Genus Lintivirus which attacks feline immune systems, leading to a greater susceptibility to infection, just as HIV does in humans.
FIV+ Cats can live long healthy lives
In 1986, a California woman who sheltered stray cats noticed that an unusual disease was spreading among some of her cats which shared cages with a specific feral cat. Concerned, she contacted immunologists at UC Davis, where Drs. Janet Yamamoto and Niels Pedersen isolated the FIV retrovirus. Since that point, work has been done to attempt to combat the virus and develop an effective vaccine.
FIV itself is a retrovirus that primarily attacks T helper cells, which are part of the feline immune system. In doing so, the virus prevents the cat's immune system from fighting off other infection. Though the virus acts in the same manner that HIV does in humans, FIV typically does not lead to any sort of AIDS-like syndrome. Like HIV, there is a period where the virus is latent (not actively reproducing), but after about six years (as a maximum), the cat will actively show symptoms of the disease. The primary symptom is simply persistent infections or diseases. Specifically, FIV+ cats will have persistent gum infections and hard-to-treat diarrhea. Ultimately, FIV leads to massive organ failure. FIV+ cats may face higher medical costs later in their lives, and usually live lives that are a few years shorter than FIV- cats due to their higher susceptibility to disease. Overall, though, if care is take to prevent an FIV+ cat from being exposed to outside pathogens, the cat can live a long, healthy life.
It is important to note that FIV cannot be transmitted to any animal other than another cat*. The virus itself exists in feline blood, saliva, and cerebrospinal fluid, but cannot live outside of a cat's body for any significant period of time. Thus, the virus is primarily transmitted through bodily fluid contact, such as bite wounds, or from a mother to her kittens. However, it is occasionally possible (according to some experts) to transmit the virus through saliva or other casual contact. As a result, it is often a good idea to wash your hands both before interacting with an FIV+ cat (to prevent the FIV+ cat from catching anything you may have on your hands) and afterwards (to keep non-FIV+ cats from contracting FIV).
Blood tests are available to test for FIV. Animal shelters do them all the time to ensure that they are not spreading the infection among their animals, and most veterinarians will be able to either do the test, or recommend a lab to go to. The test involves the use of a small blood sample, and most of the time your test can be done in-house, with the results available in minutes. However, it should be noted that it can take up to two years after infection before the test can actually turn up positive. In addition, when testing kittens under four months old, false positives are possible because the kittens may have antibodies from their mothers. If a young kitten does test positive for FIV, it should be retested at six months old. If the antibodies are still present, then the kitten is most likely FIV+, but often, the test will turn up negative, indicating that the first test showed a false positive.
FIV has a rather low occurrence rate among domestic cats (between 1.5 and 3%), but the incidence is significantly higher among those cats that live on the streets (due to the higher probability of infection) and those carrying other diseases. In general, animal shelters that accept strays will test for FIV, along with FeLV and other diseases when the animal is admitted. Whether the shelter will keep the animal if it tests positive is based on individual shelter policy. Often, shelters that euthanize unadopted animals (local humane societies, for example), will also euthanize FIV+ cats simply because they cannot care for them. Other shelters will actively take FIV+ cats and let them stay for their entire lives.
There is also an FIV vaccine available. Released in 2002 by Dr. Yamamoto (one of the original discoverers of the virus, now at the University of Florida) and Fort Dodge Animal Health researchers, the vaccine was developed using two of the five "strains" of FIV (Clades A and D), and was tested against Clade A. However, the most common strain seen in the U.S. is Clade B, which the vaccine has not been tested against. Also, because of the way the vaccine works, all vaccinated cats will test positive for FIV for the rest of their lives. The false positive may lead to the cat being euthanized if it gets lost and is turned in to many animal shelters that do not house FIV+ cats. In addition, the effectiveness rate of the vaccine is about 80%, meaning that if a vaccinated cat that is later exposed to FIV it has a one in five chance of contracting the virus without the knowledge of its owner. Finally, the vaccine uses adjuvants, which speed up the production of antibodies, but have also been shown to cause tumors in cats. As a result, many feline health care professionals are reluctant to use the vaccine.
I personally have worked with FIV+ cats at a shelter in Chicago, and know people who have many FIV+ cats of their own. Though I'd like to say that there is absolutely no difference between these cats and others that really isn't the case. At the shelter, the FIV+ cats got their own room in order to prevent the spread of the disease. In addition, the shelter had very strict rules on adopting FIV+ cats out (essentially, an adopter would need to show that they were committed to caring for an FIV+ cat and that no non-FIV+ cats would ever be brought into the household). However, behavior-wise, these were some of the nicest cats I have ever met. Because it was a cageless shelter, the FIV+ cats were able to interact with each other, and I was always surprised with how well they got along together. On the main adoption floor, you could always find a number of cats that caused trouble constantly; this was never the case in the FIV room. Even the cats that were incredibly shy around people could always be seen cuddling up with another FIV+ cat. On the flip side of the issue, though, FIV+ cats that contracted infections would be caged for much longer than FIV- cats that got sick, and the adoption rate for FIV+ cats was much lower, so it wasn’t uncommon for FIV+ cats to live out their lives in the shelter. Overall, I would say that someone who is committed to being a cat owner would have no problem adopting and caring for an FIV+ cat, just so long as you don't have any FIV- cats currently, and are willing to care for a cat that you know will be sick when it gets older.
* The only two cat diseases that humans can contract are ringworm (a skin fungus), and Cat Scratch Disease or Cat-Scratch Fever (a bacterial skin infection).