During this stage the hemolymphatic system and intestines become infected. If the cat's immune system does not fight off the virus, then it progresses to: Stage Five: The bone marrow becomes infected. At this point, the virus will stay with the cat for the rest of its life. In this phase, the virus replicates and is released four to seven days later in infected neutrophils, and sometimes lymphocytes, monocytes, and eosinophils (all white blood cells formed in the bone marrow). Stage Six: The cat's body is overwhelmed by infection and mucosal and glandular epithelial cells (tissue that forms a thin protective layer on exposed bodily surfaces and forms the lining of internal cavities, ducts, and organs) become infected. The virus replicates in epithelial tissues including salivary glands, oropharynx, stomach, esophagus, intestines, trachea, nasopharynx, renal tubules, bladder, pancreas, alveolar ducts, and sebaceous ducts from the muzzle. Transmission Cats infected with FeLV can serve as sources of infection of FeLV-A. Cats can pass the virus between themselves through saliva and close contact, by biting another cat, and (rarely) through a litter box or food dish used by an infected cat. Once a cat has been infected with FeLV-A, additional mutated forms of the original FeLV-A virus may arise, as may FeLV subgroups B, C and T. In addition to domestic cats, some other members of Felidae are now threatened by FeLV (e.g. lynx and Florida panther). Overwhelming epidemiologic evidence suggests FeLV is not transmissible to either humans or dogs. Approximately 0.5% of pet cats are persistently infected with FeLV, but many more pet cats (>35%) have specific IgG antibodies which indicate prior exposure and subsequent development of immunity instead of infection. FeLV is highly infectious. Kittens can be born with it, having contracted it from their mother while in utero. Infection is far higher in city cats, stray or owned, than in rural cats: this is entirely due to the amount of contact the cats have with each other. Diagnosis and prognosis Testing for FeLV is possible with ELISA tests that look for viral antigens, free particles found in the bloodstream. These ELISA tests use blood samples most often, but can also use saliva or eye secretions. The sample is added to a container or dish that contains the antibodies to the viral antigens. If the antigens are present in the sample, the antibodies will bind to them and an indicator on