The acquired immunodeficiency syndrome (AIDS) epidemic is now recognized as the most serious disease pandemic of our time. An all-out research assault on this deadly disease, unprecedented in scope and extent, is being conducted throughout the world, and new findings are surfacing with startling rapidity.

Sexually Transmitted Infections

human immunodeficiency virus (HIV)

The immune-system-destroying virus that causes AIDS.

At the end of the three decades since AIDS was first identified, researchers world­wide are pursuing ways to improve prevention and treatment of this disease while con­tinuing to seek a cure (Dieffenbach & Fauci, 2011). We address these unprecedented efforts in the following pages.

AIDS results from infection with the human immunodeficiency virus (HIV). HIV falls into a special category of viruses called retroviruses, so named because they reverse the usual order of reproduction within cells they infect, a process called reverse transcription.

Two forms of HIV have been linked with the development of AIDS: HIV-1 and HIV – 2. HIV-1 was the first human immunodeficiency virus to be identified and is the one that causes the greatest number of AIDS cases in the United States and throughout the world (De Cock et al., 2011). HIV-2 occurs in some African countries along with HIV-1. HIV – 1, the more virulent of the two forms, is a formidable enemy because it is constantly mutat­ing and is present in multiple strains or subtypes (Osborn, 2008; Taylor et al., 2008). To simplify our discussion of AIDS, we refer to the infective agent simply as HIV.

A great deal of speculation about the origin of AIDS has occurred since the emer­gence of the global pandemic. It has been variously proposed that HIV came from resi­dents of Africa or Haiti, mosquitoes, monkeys, pigs, or even from early testing of a polio vaccine in Africa in the 1950s. Recently published research appears to have solved the riddle of the origin of HIV/AIDS. Persuasive evidence that HIV was introduced to humans from chimpanzees was obtained by two international teams of scientists who traced the roots of HIV to a related virus in a subspecies of chimpanzees that reside in central and southwestern Africa (Gao et al., 1999; Keele et al., 2006). Genetic analysis revealed that this subspecies, Pan troglodytes troglodytes, harbors a simian immunode­ficiency virus (SIV) that is the origin of HIV-1. Evidence suggests that SIV is quite ancient, at least 32,000 years old (Worobey et al., 2010). Scientists believe that SIV genetically converted to HIV either while it was still in a chimpanzee or after a human contracted SIV, perhaps through exposure to chimpanzee blood from hunting or han­dling the meat during food preparation (De Cock et al., 2011).

Armed with evidence implicating a specific subspecies of chimpanzees in the origin of HIV, another research team conducted tests that allowed them to estimate that HIV first evolved from the SIV carried by these chimpanzees sometime between 1915 and 1941, with 1931 being the most likely year (Korber et al., 2000). With such an early date of origin, why was HIV not identified as the AIDS-causing virus until 1983? Sci­entists believe that when SIV turned into a human killer, probably in the early 1930s, it likely remained confined to a small population in an isolated area, such as a village, until migration into large cities and jet travel spread the virus worldwide. Evidence that HIV existed well before its identification was provided by discovery of HIV in a frozen blood sample collected in 1959 from an adult male residing in Africa (De Cock et al., 2011; Zhu et al., 1998). Thus it now appears likely that HIV originated early in the 20th century by means of cross-species transmission from a subspecies of chimpanzees to humans, and then was spread worldwide much later, when Africa became less isolated.

HIV specifically targets and destroys the body’s CD4 lymphocytes, also called T-helper cells or helper T-4 cells. In healthy people these cells coordinate the immune system’s response to disease. The impairment of the immune system resulting from HIV infections leaves the body vulnerable to a variety of cancers and opportunistic infections (infections that take hold because of the reduced effectiveness of the immune system). Initially, HIV infection was diagnosed as AIDS only when the immune system became so seriously impaired that the person developed one or more severe, debilitat­ing diseases, such as cancer or an unusual form of pneumonia caused by the protozoan Pneumocystis carinii. However, effective January 1, 1993, the CDC broadened this defi­nition of AIDS to include any HIV infection in which the immune system is severely

CHAPTER 15

impaired. Now anyone who is infected with HIV and has a CD4 count of 200 cells or less per microliter of blood is considered to have AIDS, regardless of other symptoms. (Normal CD4 counts in healthy people not infected with HIV range from 600 to 1,200 cells per microliter of blood.)