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.
Feline leukaemia virus (FeLV) belongs to the retrovirus family of viruses, in a group known as oncornaviruses. Oncornaviruses can cause the development of cancers, among other diseases (in both people and animals). FeLV only causes disease in cats and was first discovered in 1964.
Feline leukemia virus (FeLV) is one of the most common infectious causes of disease of cats globally. Infection with FeLV can cause a variety of clinical signs, impacting a cat's longevity and quality of life.

Furthermore, visual representations like the one above help us fully grasp the concept of Feline Leukemia And Young Cats Stress Levels.
ABCD cats & vets: GUIDELINE for Feline Leukaemia Virus Infection.
Feline leukemia virus (FeLV) is one of the most common and important infectious diseases in cats, affecting between 2-3% of all cats in the United States and Canada. Infection rates are significantly higher (up to 30%) in cats that are ill or otherwise at high risk (see below).

Lakeville Animal Hospital's, Dave Wolfthal, tells you all you need to know about feeding your cats, Feline Leukemia and AIDS, and Lyme Disease in your dog.
Feline Leukemia Virus (FeLV) is a common but preventable virus that can affect almost all cats. Dr. Boldan shares how FeLV weakens a cat's immune system, symptoms, and how the feline leukemia virus vaccine can be life-saving.