Let me give you an overview of how it was presented to me in my animal diseases class.

A cat who is exposed to FeLV can test positive. That doesn't mean that cat will contract the disease. There are three possibilities.

First, many cats are exposed, fight it off, and later test negative. So there is a period of a month or two where they will test positive because the antibodies are present in their system.

This is why none of the vets I worked for considered a positive FeLV test to be a death sentence; they always re-tested three months later. The cats who later test negative have developed immunity due to their exposure to the virus. It is very common for young outdoor kittens to be exposed and test positive but later (and forever after) test negative.

Second, some cats are exposed, never become sick, but always have antibodies present when tested. These cats can be contagious.

Third, obviously, some cats do become sick.