Feline Immunodeficiency Virus also referred to as FIV, is a lentivirus. Lentivirus is a class of viruses that cause chronic and deadly diseases that last for a very long time. For instance, the human form of a lentivirus is HIV which causes AIDS. Many mammals including humans get lentiviruses. In cats, the virus attacks the immune system. Neither Humans nor cats can infect one another with their versions of lentivirus.
FIV infected cats will sometimes appear normal for a long time. Yet, it can ultimately debilitate the immune system. The infected cat will have no defense against viruses like Calicivirus, Parvovirus and others. Not to mention coccidia and giardia. Then there is fungi in the environment and enhanced allergic reactions. This can cause illnesses that may be quite severe.

Treatment
Owners may be able to keep the disease at bay by treating the secondary infections. These may pop up, due to the weakened immune system. Also, consider other household cats living in the same environment.
Transmission
Deep bite wounds transmit the disease from one cat to another. and it can also cross the placenta from mother to unborn baby kittens. Many outside cats and cats in colonies have feline immunodeficiency virus. Cats from shelters are also at risk.
Diagnosis
To diagnose feline immunodeficiency virus veterinarians look for:
symptoms of lethargy, fever, anorexia, swollen lymph nodes. Additionally, a blood test to look for antibodies. False positives can occur though because the cat can carry antibodies. Kittens may have antibodies from Mother’s milk and yet not have the virus.
There have been vaccines developed but none have proved to work consistently. Again cats can show positive results on FIV testing. Negative blood test results are normally a better sign. It may show the cat’s body has not had the chance to mount a defense against the disease.
More recently the team at UC Davis PCR lab have been able to pcr test for the presence of the FIV virus by looking at DNA samples. For the time being though there is no cure for FIV and scientists are working to diagnose and help us to find that cure.