My Shadowfax tested positive for FIP, which is fatal just like FeL. I had a prayer thread on it here on FR. It turned out to be a false positive. I agree that clearly fatally ill cats should be put down, but you shouldn't put your cat down immediately because a test says so.
I didn't put her down immediately - thus, this thread. She is definitely a sick kitty, though, and not to mention the positive FeL test to a potential adoptee would be inexcusable.
FIP is much less reliable a test that FeLV, false positives are common in FIP. FIP is a mutant of a virus most cats carry. So when the cat tests positive and is sick, the presumptive diagnosis is FIP, when really the cat may just be a normal carrier of the virus and happen to be sick.
The good news with FeLV is that it is a disease that can be vaccinated against with high efficacy. The majority of cats that "cook" the virus for a while destroy it and live a happy, long, leukemia-negative lives.
I don't make this stuff up:
http://www.merckvetmanual.com/mvm/index.jsp?cfile=htm/bc/56600.htm&word=feline%2cleukemia
The important line to remember in all of this is: "In ¡Ý70% of adult cats, viremia and virus shedding are transient, lasting only 1-16 wk." This kitten is only a few weeks old, she has a good chance of being in that 70%.
(private note to Zia's mom - apologies, I remembered 60% from my classes, Merck says 70%)