Feline leukemia virus (FeLV) is a retrovirus that infects cats. FeLV can be transmitted by infected cats via the transfer of saliva or nasal secretions. If the animal's immune system fails to neutralize the virus, it may become severely weakened by it, potentially leading to lethal disease. Because FeLV is cat-to-cat contagious, FeLV+ cats should only live with other FeLV+ cats.