One explanation of Nef gene behavior

The Nef (negative factor) gene of primate retroviruses may serve an important evolutionary function. Selection pressures in the natural world, may at times, demand that retroviral infection not cause disease in a newly entered host species. Genetic alterations in the Nef gene may function to permit retroviral speciation by lowering retroviral replication rate. Slow viral replication may allow small numbers of the newly infected host species to avoid the effects of retroviral pathology.