Feline leukemia virus (FeLV) is a retrovirus, a family of viruses which has many members that infect cats and cause disease and death in cats around the world. Another well-known feline retrovirus is the feline immunodeficiency virus (FIV). FeLV and FIV can be found together in the same cat. The prevalence of FeLV in single-cat households is about 3% and can be as high as 11% in stray or high risk cat populations. In large multi-cat households and in households where cats roam freely outdoors, the prevalence can reach as high as 70%. Cats roaming in urban areas are more likely to be exposed to FeLV (40%) than cats roaming in rural areas (6%). FeLV has been studied for over 30 years, both for its relevance to the cat population and because it serves as an animal model for some human diseases. Research has established key characteristics of FeLV: it is contagious, it directly causes both fatal cancerous and non-cancerous diseases, it can lie dormant in the bone marrow for a long time, and it can be protected against by vaccination. FeLV is not transmissible to humans or animal species other than the cat family.