Implications for Origins of HIV-1
Two possibilities for the origin of HIV have been widely discussed. One possibility is that the virus has always been present in the human population, in one form or another, but may have gone unrecognized due to an extremely low prevalence and/or its confinement to isolated populations. In this model, features of modern society, urbanization of populations in developing countries, and extensive world travel would have introduced the virus into developed countries. The virus would then be spread by sharing of drug paraphernalia and sexual promiscuity. Alternatively, HIV-1 may have entered the human population from another species relatively recently. Increasing prevalence and high mortality would then be consequences of infection of the new host to which the virus was not fully adapted. The most likely possibility is a combination of these two: that the virus has been entering the human population from an animal reservoir (through the butchering of monkeys, for example) but has been unable until recently to spread from initial limited, foci of infection. Which theory is correct has not been resolved and may never be. The most significant clue has been the discovery that a few captive chimpanzees are infected with SIVcpz, a virus in the HIV-1 group (Fig. 1) (Huet et al. 1990). However, wild chimpanzee populations are difficult to study, and the vast majority of chimpanzees studied to date are not infected with SIVcpz. A major question is whether the few SIVcpz isolates represent a natural infectious agent of chimpanzees.