Feline leukemia virus (FeLV) is a retrovirus that infects cats. FeLV can be transmitted between infected cats when the transfer of saliva or nasal secretions is involved. If not defeated by the animal’s immune system, the virus can be lethal. The disease caused by this virus is a form of cancer of blood cells called lymphocytes (a leukemia). Stage One: The virus enters the cat, usually through the pharynx where it infects the epithelial cells and infects the tonsorial B-lymphocytes and macrophages. These white blood cells then filter down to the lymph nodes and begin to replicate.