General Information

Currently, there are over 100 known retroviruses that exist amongst seven different genera. The viruses used to be classified and divided into genera based upon the morphology of the virion core as visualized under electron microscopy (EM), as well as other defining biological and physical characteristics. However, due to the increasing amount of retroviruses that have been discovered, the older classification system is no longer sufficient. Instead, the retroviruses are now divided into the following genera based on sequence data: alpha, beta, gamma, delta, epsilon, lenti, and spuma. Only the genuses delta, lenti, and spuma contain retroviruses that infect humans:

• Human Immunodeficiency Viruses (HIV)
• HIV-1 (lenti)
• HIV-2 (lenti)
• Human T-Lymphotropic Viruses (HTLV)
• HTLV-1 (delta)
• HTLV-2 (delta)
• HTLV-3 (delta)
• HTLV-4 (delta)
• Human Spuma Retrovirus (HSRV)
• HSRV (Spuma)