I don't think there is a cure if the cat has full-blown FeLV. The way I
understood it, if a cat has recently been exposed (esp. a kitten), they
can "throw" the disease. I googled for a better explanation. This is
from a website I wanted to share because it's almost exactly the way my
vet explained it to me and it kind of gives hope. Also, the test is
notoriously inaccurate. It bugs me to think how many cats are given up
on from just that one snap test.

"A positive test result means that a cat has been exposed to the feline
leukemia virus.  It's a good idea to confirm a test done in the office,
which is the common procedure here in the States, with a different test
done at a lab facility.  Some cats exposed to the virus will throw it
off and become negative but you usually don't notice that and, by the
time  the cat is tested, it may just be negative.  But they might also
want to  test again in 3 months to see if the cat has gone from
positive to negative.

This doesn't mean that the cat has leukemia.  That name was given to
the virus several decades ago.  Actually, cats positive for the virus
are  more susceptible to other diseases, like respiratory disease, GI
problems,  etc. Also lymphoma.  All because the virus affects how their
immune system functions.

If a cat remains positive, it can still lead a good life but the
chances are that it won't live as long as a healthy cat.  But with good
nutrition, living indoors so there isn't a lot of exposure to
unhealthy stuff, jumping on any illnesses asap, and, perhaps, using
drugs like  interferon alpha or Immunoreglan to stimulate the immune
system,
these cats can do well.  This virus is fairly easy to pass from cat to
cat.  (Unlike FIV which is hard to pass.)  So a positive cat should
really not be exposed  to other cats.  There's a vaccine against FeLV
but it isn't 100%  effective."

From:
http://www.newton.dep.anl.gov/askasci/vet00/vet00055.htm