FIV is a complex retrovirus of the genus Lentivirus. There are at least five different subtypes (clades; A, B, C, D, E) of FIV found in domestic and wild felids throughout the world. FIV primarily infects lymphocytes and integrates its genetic information into the genomic DNA of infected lymphocytes. Over time, there is a selective and progressive loss of CD4+ T-helper lymphocytes, which primarily results in impaired cell mediated immunity.