It is very common in feral colonies and outdoor cats who have never been vaccinated. The vet is recomending the test because she came into contact with cats who may have had the virus at the shelter. It is very similar to human Aids as it can supress the immune system. Cats who contract leukemia in thier teen years stand a much better chance than a kitten for survival. It does not mean a death sentence, but it means you watch for upper respitory infections because they can't fight them very well. It also means that if you decide to care for a leukemia + cat, then you need to keep the cat indoors and away from other cats.