Feline leukemia virus is a retrovirus that infects cats. FeLV can be transmitted from infected cats when the transfer of saliva or nasal secretions is involved.
FeLV is a type of virus called a retrovirus, meaning it can be incorporated into a cats genome and may not be cleared over time. Persistently infected cats shed infectious viral particles in saliva, as well as in feces, urine and milk.

Alley Cat Allies is committed to providing cat owners, caregivers, veterinarians, veterinary technicians, policymakers and anyone who cares for cats with the most up-to-date facts about Feline Leukemia Virus (FeLV).

Feline leukemia is a disease that only affects cats -- it cannot be transmitted to people, dogs, or other animals. FeLV is passed from one cat to another through saliva, blood, and to some extent, urine and feces. The virus does not live long outside the cats body -- probably just a few hours.

The virus is spread in saliva, urine, feces, nasal secretions and milk from nursing mothers. When an infected cat bites or grooms another cat, that cat may develop the virus. If a pregnant cat has feline leukemia, the kittens might be born with the disease or may develop it after nursing.
ABCD cats & vets: GUIDELINE for Feline Leukaemia Virus Infection.