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INTRODUCTION

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AIDS emerged as one of the most important public health issues of the late twentieth and early twenty-first centuries and is now one of the leading causes of global morbidity and mortality. The AIDS epidemic has prompted wide-reaching changes in public health, clinical practice, and scientific research, and has had a great impact upon societies throughout the world.

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History and Origin
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AIDS was an unrecognized medical syndrome before the 1980s.1,2 In the United States, investigations into the increase of opportunistic diseases occurring in previously healthy young adult males resulted in the first reports of AIDS among homosexual men in 1981.3,4 In Africa, AIDS was initially described in both men and women in 1984,5,6 with the earliest known HIV-1 seropositive specimen identified through retrospective studies from Central Africa in 1959.7 HIV-2, a related retrovirus that is less transmissible and pathogenic but that can still cause AIDS, was first isolated in West Africans in the mid-1980s, with the earliest known recorded HIV-2 infection probably having occurred in Guinea Bissau in the 1960s.8 Based on molecular analyses comparing human and simian (monkey) retroviruses, it is believed that the HIV-1 virus that became epidemic was first transmitted to humans from chimpanzees in Central Africa in the 1930s, and that the HIV-2 virus was first transmitted to human from sooty mangabeys in West Africa.9,10,11,12

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Both social and biologic factors explain why AIDS initially became epidemic in the industrialized world after HIV was introduced in the population. Among these were changes in sexual behaviors, particularly increases in the numbers of sexual partners and sexually transmitted diseases among substantial numbers of men who have sex with men, high rates of injection drug use (both opiates and cocaine), and sharing of contaminated needles and syringes, and the development of technology to use large plasma pools with thousands of donors for manufacture of clotting factor concentrates. These factors represented amplification systems that, in combination with the long incubation period for AIDS, allowed extensive transmission of HIV to occur even before the first clinical cases were discovered. In Africa, a number of factors are hypothesized to have contributed to the rapid early spread of HIV in the 1980s, including rural-to-urban migration patters, the establishment of HIV infection in female sex workers with spread to their male clients, and subsequent extensive sexual networking with both regular and casual partners.13,14 A number of important biologic risk factors for HIV transmission in Africa were identified, including high HIV viral load, young age at first sexual intercourse among women, age differential between spouses, herpes simplex type 2 infection, trichomoniasis, and lack of male circumcision.15,16 Spread between geographically distant regions was attributed to highly mobile populations such as truck drivers, seasonal workers, and migrant populations.17,18 The epidemic then spread globally ...

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