This article is from the Feline Leukemia (Cats) FAQ, posted to rec.pets.cats newsgroup. Maintained by Erin Miller with numerous contributions by others.
This is a tough situation, for which there is no pat answer. You should discuss the matter with your vet. One vet I spoke with felt that chances are the other cats have already been exposed and it is probably best to just keep them up on their vaccinations and not change the household drastically. One study (Barlough, 1984) says that in a survey of 45 households from which FeLV+ cats were removed, 99.5% of the FeLV- cats remained negative. However households in which the FeLV+ cats were not removed had infection rates 40 times greater. It is probably best to remove the infected cat if it can be sent to a good home without causing too much disruption in your household, and your mental psyche.