These diseases are not curable, but cats can potentially live long and healthy lives after testing positive for teh disease. Unfortunately, in order for your cat to live a long healthy life, you'd have to catch the presence of the disease BEFORE the cat showed symptoms. Once the cat is showing signs of illness, the symptoms can be managed to a point, but at that point, the disease will almost always prove fatal.
Both of these diseases are retroviruses. THey attack the cat's immune system, leaving them powerless to secondary bacterial and viral infections. Cats don't die OF FIV of FeLV, they die due to complications from other illnesses because they can't heal. FeLV is commonly associated with blood cancer and lymphomas. I recently lost an FeLV+ kitten to, according to the necropsy, a malignant tumor in her chest due to the complications of FeLV.
The best thing for these cats is a) to stay inside no matter what and b) to receive frequent immunotherapy treatments (preferably both traditional and holistic) and c) IMMEDIATE vet care if even the slightest symptoms appear. Immunotherapy, at least for my kitten, was in the form of Interferon-Alpha, but I have heard of people using Immunoregulin as well. Often, positive kitties have theri diets supplemented with L-Lysine, CoenzymeQ10 and Vitamin C.
Neither of these disease can be transmitted to humans. Hypothetically, the virus for FeLV could replicate in human tissue, but healthy people fight it off right away. Someone with AIDS or a very elderly person or a brand new infant should probably not be around these cats, although there are no documented cases of FeLV actually causing anything in people. At this point, it's just hypothetical lab models showing this.
FIV is transmitted through sexual activity and deep bite wounds. FeLV, on the other hand, can be transmitted through grooming, touching noses, sharing food bowls and sharing litter pans. Fortunately, it's also a weak virus, so it dies off in the environment within a couple hours. There is a safe vaccine for FeLV (it's made by Merial) and it will not cause the cat to test postive, nor will it cause the cat to give unvaccinated cats the disease. The FIV vaccine is availible, but not generally reccomended. The best course of action is to keep positive and negative cats away and keep all cats indoors so they can't catch it fro mfree-roaming animals. Also, good sanitation of the home is key.
Hope some of this helps!