At the Veterinary Clinic where I work the Vet recommends the FeLV (Feline Leukemia) vaccine for any cat that goes outside at all. It doesn't matter if the owner sees other cats around or not. It's better to be safe than sorry. All it takes is one infected stray showing up and getting into a fight with your cat. Then it's too late to vaccinate because your cat is aready infected. (He also recommends it for indoor only cats if they have a housemate that is an indoor/outdoor cat). Feline Leukemia is a virus that affects the immune system. There is no cure for it. A cat can have it for up to 3 years without showing any symptoms at all. It's transmitted via bodily fluids. Cats can get it from grooming each other, sneezing on each other, biting each other, and less commonly sharing food/water bowls and even from sharing litterboxes. Kittens can be born with it if the mother has it (they can get it through the placenta or from nursing). The FeLV vaccine is about 80% effective in preventing Feline Leukemia. Another thing the clinic where I work at recommends is testing your cat for FeLV/FIV (Feline Leukemia and FIV) before we vaccinate them for FeLV. FIV is similar to HIV in people. It's another disease that affects the immune system and there is no cure. FIV is mainly transmitted via bite wounds (mostly male cats that aren't neutered and get into a fight and bite each other). Kittens can also be born with it if the mother has it though. There is a FIV vaccine that's been on the market for a few years, but our clinic does not use it for two different reasons: Any cat that gets the FIV vaccine will test positive for FIV next time it's tested. If someone's cat that had been previosly vaccinated against FIV ever got out and ran away and ended up in a shelter that routinely tested for FeLV/FIV it would test positive for FIV and most likely end up being euthanized (because the shelter would have no way of knowing if the cat tested positive because it actually had FIV or because it had been vaccinated against FIV). The second reason we don't do the FIV vaccine at our clinc is because their is apparently 4 different strains of it (seen in different parts of the country) and each vaccine only covers certain strains. Someone could vaccinate against FIV, and actually have their cat come down with a different strain that they weren't protected against. The Feline Leukemia vaccine, however, does not make cats that are later tested for the disease show up as positive.