Feline Leukaemia Virus (FeLV) is a contagious retrovirus from the Mammalian Type-C group, which is endemic in many areas of the world. The prevalence of FeLV is particularly high among dense populations.
Transmission occurs essentially by contact, mainly through saliva or nasal secretions and by biting or licking. Clinically healthy persistantly viraemic cats are known as a major source of the virus.

Infection is characterised by the development of a viraemia which can be followed by seroconversion with elimination of the pathogen. Infected cats may also present a chronic, persistent viraemia which will lead to the development of both proliferative syndromes like lymphosarcoma or leukaemia and non-proliferative syndromes such as anaemia or immunosuppression, followed by death at short or middle term.

Diagnosis of FeLV infection is generally performed by the detection of a viral antigen from the core protein, p27, which is produced in high quantities in viraemic cats.
Feline Immunodeficiency Virus (FIV) is a lentivirus of worldwide distribution, first isolated in 1986 from several cats exhibiting signs of immunodeficiency. Prevalence rates vary from less than 1% to more than 20%, depending on the cat population (sex, age, behaviour), health status, and geographical area.

FIV is most commonly transmitted by biting. The infection is firstly expressed by a transient primary illness lasting several weeks, followed by a long period of apparent normal health which precedes a terminal immunodeficiency phase characterised by ailments such as generalised lymphadenopathy, stomatitis, gastro-enteritis, anaemia and neurological disorders. Affected cats will finally die from the development of a variety of secondary opportunistic infections or tumours.

Infection by FIV is generally accompanied by a rapid rise of antibody levels, particularly those directed against the transmembrane protein of the virus, gp40. The presence of anti-gp40 antibodies indicates that the cat has been exposed to the virus, even in the case of complete absence of symptoms.