Defined as Feline Leukemia Virus, this virus is transmitted through the saliva, feces, milk, and urine of an infected cat. Testing and vaccination recommended for all cats of all ages.
Your cat may remain asymptomatic for years after developing a small fever intially. As the cat's immune system declines, it causes degenerative diseases such as anemia, liver disease, intestinal disease and reproductive problems; cancerous diseases such as lymphoma or leukemia; and diseases associated with immunosuppression, including increased susceptibility to infectious agents. Chronic respiratory infections, chronic gingivitis and stomatitis (inflammation of the gums and mouth), poor healing of wounds and abscesses and other infections are common in cats infected with FeLV.
Best form of prevention is testing for infection and vaccination.
FeLV-infected cats are found worldwide, however infection depends on age, health, environment, and lifestyle. In the United States, approximately 2 to 3% of all cats are infected with FeLV. Rates rise significantly—13% or more—in cats that are ill, very young, or otherwise at high risk of infection. http://www.vet.cornell.edu/FHC/health_resources/brochure_felv.cfm
For more information on FeLV
Cornell University College of Veterinary Medicine - prevalence, treatment and symptoms of FeLV