According to http://www.winnfelinehealth.org/Pages/FeLV_Web.pdf:
FeLV is responsible for the illness and death of more cats than any other disease condition.
Vaccination is recommended only for those cats whose lifestyle places them at risk for FeLV. This includes outdoor cats or those that are indoor/outdoor, feral cats, cats in open multi-cat households, cats in FeLV-positive households, and cats in households where the FeLV status of all resident cats is not known.
And according to http://www.feline-leukemia.net/disea...line-leukemia/:
Statistics say that one out of ten cats is a Feline Leukemia carrier. Feline Leukemia is a virus that is dreaded by every cat owner around and this is understandable since it is said to be one of the prime causes of cancer in cats.
It is also labeled as a top cat killer, apart from being hit by fast-running automobiles. The death of an infected cat is more of a death out of complications developing through this disease because of the weakened immune system. Some infected cats are dying fast due to rampant infections or colds, rather than the virus contracted via the feline leukemia. Like the human AIDS, this disease can show off symptoms only after years of acquiring the virus and are therefore hard to counter because of retrograde signs.