FIV is the Feline Immunodeficiency Virus.
Virologists classify feline immunodeficiency virus (FIV) as a lentivirus (or "slow virus"). FIV acts slowly, over a course of years, to erode a cat's immune system. It cannot be passed to humans, or dogs or any other animal than cats, and cat-cat transmission is possible only through deep bite wounds, infected gums, etc. It cannot be transferred by licking, sharing a litter box, etc. FIV+ cats can live long and healthy lives.
Please read more about FIV in this page from Treehouse
More good info from Best Friends
FIV Quick Facts:
FIV is not communicable to humans, dogs or other species.
FIV+ cats are not sick. FIV is a virus that causes their immune systems to be compromised, but is not an illness itself.
FIV+ cats' life expectancies are not significantly shorter than other cats that do not have FIV. With good care, they can live as long as any other cat.
Adopting an FIV+ cat, does not mean that you will be spending more money on medical expenses than a non positive cat.
FIV is transmitted from one cat to another by contact with bodily fluids, such as fighting among males that have not been neutered. Intact tomcats fight to hurt each other and inflict deep puncture wounds -- this is where the virus is spread. Normal playing, or even the occasional skirmish, is highly unlikely to cause infection.
All cats require a high quality diet; FIV cats are no different. A high-quality diet will help your cat stay healthy.
Some people believe FIV+ cats should not be vaccinated, since a vaccine is a "hit" to the cat's (already less than 100%) immune system. Please consult your veterinarian.
As with any cat, FIV+ cats should be examined by a veterinarian annually, and get regular blood work and dentals as needed.