Feline immunodeficiency virus (FIV) is a member of the lentivirus subfamily of retroviruses. This group of viruses, which includes the human and simian irnmunodeficiency viruses (HIV and SIV) derive their name 'from the typically slow time course of the infections they cause. 15 FIV is a typical lentivirus with respect to gross and structural morphology. The viral genome is composed of single-stranded RNA which is transcribed into double-stranded RNA which is transcribed into double-stranded DNA using a Mg2+ dependent reverse transcriptase enzyme. The viral DNA is then inserted into the genome of the infected host cell and is replicated whenever the host cell divides. Lentiviruses are known for persistent viral infections within mononuclear cells, often in the face of a strong host immune response.