FIV stands for Feline Immunodeficiency Virus. This virus is in the same family of viruses as FeLV (Feline Leukemia Virus). They are both retroviruses. But the subfamily of FIV is different: It is a lentivirus (or “slow virus”). The other familiar lentivirus to all of us is HIV, which causes AIDS in humans. So, humans are not alone in having a virus that affects the immune system.