Feline leukemia vaccine is a core vaccination in kittens, but a lifestyle vaccination in adult cats. It is a contagious virus that is spread between cats through contact with infectious fluids, typically saliva. Common ways of transmission include grooming, fighting, sharing of food/water, and litter box use. . The vaccine can be administered to kittens as young as 8 weeks of age, or unvaccinated adult cats with exposure risk, in a 2-part series, then boosted 12 months later. After the first 12-month booster, cats with exposure risk should be vaccinated every 1-2 years. It is recommended that your cat be tested for leukemia through a blood test prior to administering the vaccine. Fighting between cats is the most common way this potentially deadly virus is transmitted, as a result, owners of vaccinated and unvaccinated cats should try to prevent interactions between unfamiliar cats.