I had two cats that were diagnosed with Feline Leukemia, which back then was also known as Feline AIDS. They were two years old at diagnosis, and they lived until they were 17-18 years old.
While there is some confusion between two deadly diseases a few decades back, Feline Leukemia is not the same as Feline Aids(or FIV).
Feline Immunodeficiency Virus: http://www.vet.cornell.edu/FHC/health_information/brochure_fiv.cfm We now know these cats, with proper care, can live very long and normal lives. Even being put together with non-FIV cats as the virus is spread through deep bite wounds.
Feline Leukemia Virus;http://www.vet.cornell.edu/fhc/Health_Information/brochure_felv.cfm Although related, FeLV and FIV differ in many ways, including their shape: FeLV is more circular while FIV is elongated. The two viruses are also quite different genetically, and their protein constituents are dissimlar in size and composition. Although many of the diseases caused by FeLV and FIV are similar, the specific ways in which they are caused differs.
I have an FIV positive cat and anticipate he will live a long and happy life. He shares his home with several female cats who are not infected.
That said, if a cat that was FeLV positive were available for adoption, I would have to pass as the FeLV is EASILY transmissible to other cats as the FIV is not easily transmitted.
**If the FeLV cat is to be an indoor only cat and the only cat in the home an adoption might be doable. Although adopting one such cat is to take on a host of future health issues. Also if the cat does escape the home, they could potentially spread the disease very easily.