Feline Immunodeficiency Virus (FIV) is a virus specific to the cat family. It was first recognized in the mid 1980's and it has been found in cats worldwide. Although widespread, it is not a common infection in cats. Only 1-5% of cats show evidence of exposure to the virus. In some cats exposure to the virus leads to clinical signs and symptoms that result in deficiency in the immune system.
Feline leukemia virus (FeLV) is one of the most important infectious viruses of cats. It was first discovered in cats with a form of leukemia, hence its name. FeLV is the cause of a variety of diseases, not just leukemia.