Feline immunodeficency virus, commonly called FIV,  is a retrovirus and is a member of the lentivirus family.  This virus infects domestic cats and cheetahs and is endemic throughout the world.  It attacks and weakens the immune system, making the animal susceptible to infections and diseases that do not cause problems in healthy cats.  Infected cats shed virus particles is mainly in their saliva, and the principal mode of transmission is through deep bite wounds.  Due to higher incidence of fighting and therefore being bitten, male cats are more often infected with this virus than female cats.  Simailarly cats that roam free and aged cats are more commonly infected.

Following infection, there is a transient period of fever, swelling of the lymph nodes, and reduction in circulating white blood cell numbers. Most cats recover from this first stage and appear normal for months or years before immunodeficiency occurs.  FIV attacks the immune system of cats, much like HIV attacks the immune system of human beings. FIV infects many white blood cell types in its host, including T lymphocytes, B lymphocytes, and macrophages. FIV eventually leads to debilitation of the immune system in its feline hosts by the infection and exhaustion of T-helper cells.