In animals that are known to be the natural hosts of SIV, there is no CD4+ cell
depletion and no manifestation of AIDS-like disease despite high virus titers.
The human-adapted viruses (HIV-1 and HIV-2) similarly do not show disease in
non-human primates (which causes problems in testing vaccines). But when SIV
is transferred to an unnatural host such as macaque moneys or humans, CD4+ cells
diminish and the host animal is susceptible to opportunistic infection. It is
possible that the natural hosts of SIV are resistant to disease because they are
the descendents of animals that survived epidemics of SIV.