Feline immunodeficiency virus
Feline immunodeficiency virus (FIV), commonly known as Feline AIDS is a lentivirus that affects domesticated housecats worldwide.  According to Template:Harvcoltxt, 11% of cats worldwide are infected with FIV. According to another study, 2.5% of cats in the USA are infected with FIV Template:Harv. FIV is in the same retrovirus family as Feline leukemia virus (FeLV). FIV differs taxonomically from two other feline retroviruses, feline leukemia virus (FeLV) and feline foamy virus (FFV). Within FIV, five subtypes have been identified based on amino acid sequence differences coding for the viral envelope. FIV is not a death sentence for cats, but is also not curable. A vaccine is available although its efficacy remains uncertain.
FIV was first discovered in 1986 in a colony of cats that had a high prevalence of opportunistic infections and degenerative conditions, and has since been identified as an endemic disease in domestic cat populations worldwide Template:Harv.
The primary mode of FIV transmission is deep bite wounds, whereas FeLV is easily spread by casual contact such as grooming and shared water bowls.  Experts disagree as to whether FIV can be spread by casual contact.  The virus is also transmitted via mucosal surfaces such as those in the mouth, rectum, and vagina.
FIV attacks the immune system of cats, much like the human immunodeficiency virus (HIV) attacks the immune system of human beings.  FIV infects many cell types in its host, including CD4+ and CD8+ 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 (CD4+) cells.
FIV and HIV are both lentiviruses; however, neither can infect the other's usual host: humans cannot be infected by FIV nor can cats be infected by HIV. FIV is transmitted primarily through saliva (bites), such as those incurred during territorial battles between males. Cats housed exclusively indoors are much less likely to be infected, provided they do not come in contact with infected cats.
FIV infected cats are often euthanised. A vigilant pet owner who treats secondary infections can make a difference.  An infected cat can often live a nearly normal life span Template:Harv. The chance that an FIV infected cat will pass the disease onto other cats within a household is very low as long as there is no fighting or biting Template:Harv. Keeping infected cats separated from disease-free cats is the only sure way of preventing the spread of the disease.
The disease occurs in three stages: First is the Acute Stage (1-2 months after transmission) in which fever, depression, and generalized lymphadenopathy are observed Template:Harv. Second is the Subclinical Stage (4 weeks to X months after transmission), in which symptoms of the disease decrease or disappear; however, all cats remain viremic for life. Third is the Chronic Stage, in which cats eventually succumb to chronic infections due to suppressed immune system function. Cats may incur stomatitis, odontoclasia, periodontitis, gingivitis, rhinitis, conjunctivitis, pneumonitis, enteritis, and dermatitis in the later stages of infection.
FIV infects other feline species, and in fact is endemic in some large wild cats, such as African lions.  Unlike domestic cats,  these species do not necessarily exhibit symptoms, perhaps because they have developed evolutionary mutations that confer resistance.