heterosexuals who use crack cocaine (Centers for Disease Control, 1992f). A number of factors are associated with this rise, including multiple-partner sex to procure drugs, especially crack cocaine; inadequate health care among groups at risk; and declining educational levels among lower socioeconomic populations (Centers for Disease Control, 1991g).
The origin of HIV, the lentivirus (a member of the retrovirus family) that causes HIV disease and acquired immunodeficiency syndrome (AIDS), is not known. Viruses closely related to HIV have been present for hundreds, if not hundreds of thousands, of years in African nonhuman primates. Similar comparisons of human and nonhuman lentivirus isolates strongly suggest that HIV-2, the variant of HIV found primarily in persons from West Africa, may have infected humans following cross-species transmission from nonhuman primates (Gao et al., in press). This transmission could have occurred through bites of infected monkeys that were kept as pets, captured by trappers, or transported to other countries. Nucleotide sequence analyses show that HIV-2 and some isolates of simian immunodeficiency virus (SIV), an HIV-like virus, belong in the same subgroup; there are no data at present placing an SIV isolate from monkeys in the same subgroup as HIV-1. However, an SIV virus belonging to the same subgroup as HIV-1 has been isolated from chimpanzees. Yet if HIV-1-like viruses are present in nonhuman primates, it is possible that both HIV-1 and HIV-2 were occasionally, but infrequently, transmitted to humans and persisted in remote areas or in isolated individuals or populations for centuries. Although the earliest documented case of HIV infection was obtained from a serum sample collected in central Africa in 1959 (Garry, 1990), the country or continent in which the HIV disease epidemic began is not known. What is clear is that HIV and SIV arose through natural evolutionary processes.
The HIV disease pandemic initially escaped detection because of the disease's long incubation period (the time from infection to onset of life-threatening disease). After reservoirs of infection had been established in African and Western countries, HIV spread to all parts of the globe. In Africa, it is believed that HIV was transported by the movement of infected individuals from isolated rural communities to rapidly expanding cities. This urbanization was accompanied by changes in sexual behavior, which played a major role in the transmission of HIV among (primarily) heterosexual populations in Africa (Quinn et al., 1986). Preexisting infection with other sexually transmitted microorganisms, especially those that cause genital ulcers and local genital tract inflammation, probably facilitated heterosexual transmission of HIV (Hillman et al., 1989).
The early spread of HIV, particularly in the United States and Europe, was largely the result of high-risk sexual practices of some male homosexuals, and it was in this population that most of the early cases were seen. Within a short period, however, another major group of HIV-infected individuals