A novel Gammaretrovirus, named Koala retrovirus (KoRV) was identified in koalas in 2000. Subsequent testing has shown that KoRV appears to be widespread throughout the wild koala population in Australia. KoRV is apparently unique in that it is the only known example of an exogenous retrovirus which is actively undergoing the process of endogenisation. While there is currently little direct evidence that KoRV causes overt disease in koalas, it is possible or even likely that KoRV infection plays a significant role in the pathogenesis of various neoplasms and a variety of immunosuppressive disorders which are commonly diagnosed in koalas. It appears that KoRV may have arisen through the cross species transmission of a closely related retrovirus in a native Australian rodent, the grassland melomys. Both of these viruses are closely related to a third retrovirus, Gibbon ape leukaemia virus (GALV). GALV was first isolated in the late 1960's from captive gibbons in Thailand that were suffering from leukaemia. To date, the source of GALV remains unknown.