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R E V I E W : A I D S AIDS as a Zoonosis: Scientific and Public Health Implications Beatrice H. Hahn, 1 * George M. Shaw, 1,2 Kevin M. De Cock, 3 Paul M. Sharp 4 Evidence of simian immunodeficiency virus (SIV) infection has been reported for 26 different species of African nonhuman primates. Two of these viruses, SIVcpz from chimpanzees and SIVsm from sooty mangabeys, are the cause of acquired immuno- deficiency syndrome (AIDS) in humans. Together, they have been transmitted to humans on at least seven occasions. The implications of human infection by a diverse set of SIVs and of exposure to a plethora of additional human immunodeficiency virus–related viruses are discussed. E merging infectious diseases represent sub- stantial threats to global health ( 1 ). As such, AIDS ranks as one of the most im- portant infectious diseases facing humankind in the 21st century. Since its initial clinical descrip- tion less than two decades ago, AIDS has result- ed in the deaths of more than 16 million people worldwide ( 2 ). Human immunodeficiency vi- rus–type 1 (HIV-1), the most common cause of AIDS, has infected more than 50 million indi- viduals (including those who have died), and the rate of new infections is estimated at nearly 6 million per year ( 2 ). Equally disturbing are the uncertainties of the epidemic to come. Although sub-Saharan Africa remains the global epicen- ter, rates of infection have increased in recent times in the former Soviet Union and parts of south and southeast Asia, including India and China, where literally hundreds of millions of individuals are potentially at risk. In the United States, new waves of infection have been rec- ognized in women, minorities, and younger gen- erations of gay men. Combination antiretroviral therapy has afforded many people clinical relief, but the costs and toxicities of treatment are substantial, and HIV-1 infection remains a fatal disease. Moreover, the vast majority of infected people worldwide do not have access to these agents. Thus, although the demographics (and, in some instances, the natural history) of AIDS have changed, the epidemic is far from over; instead, it is evolving, expanding, and posing ever greater challenges. Like the epidemic, the viruses responsible for AIDS have proven to be more complicated and more unpredictable than first recognized. The AIDS viruses are members of the lentivirus family of retroviruses. As such, they have been amply demonstrated to exhibit the remarkable (and perplexing) properties of insidious disease induction, persistence, latency, variation, re- combination, and escape from immune and drug pressures. There are two distinct types of human AIDS viruses, HIV-1 and HIV-2, which are distinguished on the basis of their genome organizations and phylogenetic (i.e., evolution- ary) relationships with other primate lentivi- ruses. Both have been further subclassified on the basis of phylogenetic criteria. Current data indicate that HIV-1 comprises three distinct virus groups (termed M, N, and O), with the predominant M group consisting of 11 clades
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