Encyclopedia of Public Health | HIV/AIDS

Acquired immunodeficiency syndrome, or AIDS, is the final, life-threatening stage of infection with any of the human immunodeficiency viruses (HIV-1, its many subtypes, or HIV-2), which are transmitted from person to person sexually (including via anal, oral, and vaginal intercourse, both heterosexually and homosexually), through contact with blood (mainly via equipment used to inject illicit drugs and, rarely, via medical uses of blood), and perinatally (from mother to fetus or newborn during pregnancy, labor, and delivery, or after birth through breast-feeding).

ORIGIN AND HISTORY

HIV-1 and HIV-2 both appear to have been transmitted to humans from primates in Central and West Africa, probably to hunters or processors of carcasses of primates consumed as food (referred to as "bush meat"). Beginning as simian viruses, they became human viruses once they achieved sustained transmission from person to person. This appears to have occurred at least four times in history: three times from chimpanzees (Pan troglodytes); (possibly in the 1930s), representing the three major strains of HIV-1, and once from sooty mangabeys, representing HIV-2. Social and technological changes in Africa resulted in transmission of HIV to larger and larger numbers of adults as roads were built and river transport developed, making travel to cities, with their better economic opportunities, far easier and more rapid. A silent heterosexual epidemic occurred and spread via travelers to industrialized nations of Europe and North America, where the new syndrome was initially recognized as a distinct clinical entity in 1981, even though the number of cases then was minuscule. By 1983, epidemiologists had discerned the routes of transmission and pointed the way for laboratory investigators to identify the etiologic agents. In 1984, the laboratory culturing of HIV was described in the scientific literature, as was the first serologic test for detecting the HIV antibody, which has been used to screen blood donations since 1985. Originally given three different names by the French (1983) and two American (1984) research teams that "discovered" the virus, the name HIV was agreed upon in 1986.

EPIDEMIOLOGY

HIV-1 has spread worldwide, infecting more than 36 million people by 2001. HIV-2, which seems to be less clinically severe and possibly less transmissible from person to person, has mainly been a public health problem for West African nations. Originally epidemic in African and urban settings, HIV and AIDS are now among the most common serious infections globally, including in the Americas and Eurasia and in rural settings. All ages, racial and ethnic groups, and persons of all sexual orientations have been infected.

VIROLOGY

HIVs are all members of the family known as retroviruses, so named because of their unique method of reproduction which uses the enzyme (protein catalyst) reverse transcriptase (RT) to incorporate its genetic material (RNA) into the DNA of the infected host's cells. HIV infects specific white blood cells of the host's immune system, known as T-helper lymphocytes (often referred to as CD4+ cells), and destroys them. Even though the immune system produces millions of new CD4+ cells every day, HIV destroys them just as rapidly. The genetic material of HIV has been sequenced, providing a database useful for research on vaccine and antiviral drug development. Many subtypes of HIV-1 have been characterized, but all are transmitted via the same routes and result in the same immunodeficiency.

SYMPTOMS, DIAGNOSIS, AND TREATMENT

Persons initially infected with HIV may develop an "acute retroviral syndrome" characterized by fever, lymph node enlargement, and flu-like symptoms. If symptoms are present, they clear spontaneously, but all infected persons, both with and without symptoms, remain infected and infectious to others indefinitely. The incubation period is highly variable, averaging about a decade, but ranging from a few months or years to possibly longer than two decades. When sufficient damage to the immune system has been sustained, measured either by laboratory cell counts of the Thelper cells or by onset of opportunistic infections, the patient is said to have AIDS. Common manifestations of HIV infection include tiredness, lymph node enlargement, fever, weight loss, and yeast infections of the mouth and vagina.

HIV infection is diagnosed by laboratory detection of evidence of infection, usually identification of HIV-specific antibodies in a blood, oral fluid, or urine specimen. AIDS can be diagnosed in HIV-infected persons in several ways, based on either laboratory evidence of immunodeficiency (lowered levels of CD4+ cells), or clinically by onset of any one or more of a specific list of opportunistic diseases. Opportunistic diseases are those that occur only, or most severely, in patients whose immune systems are impaired. The most common opportunistic diseases in AIDS patients are Pneumocystis carinii pneumonia, Kaposi's sarcoma, toxoplasmosis of the brain, tuberculosis and other mycobacterial infections, and severe herpes, cytomegalo virus, and yeast infections.

As of 2001, all of the more than seventeen antiviral drugs used to treat HIV infection act by interfering with one of the enzymes that HIV needs to complete its life cycle. No treatments result in a cure for HIV infection. The antiviral drugs prevent HIV from growing and further damaging the host's immune system. Thus, the goal of treatment is to preserve the patient's health. Patients must take several antiviral drugs daily. Research on more and better antiviral drugs, and on methods to reconstitute the impaired immune system, is ongoing. A key part of treatment is the prevention of opportunistic infections with specific vaccines and antibiotics.

PREVENTION

Prevention of HIV infections is deceptively simple: Refrain from having sexual contact and from sharing drug-injecting paraphernalia with anyone who is infected. However, the rapid and continuing global spread of HIV, despite its well-known and severe clinical consequences, points out how difficult it is to change risky sexual and drug-taking behaviors. Many successful educational and social interventions have been demonstrated, but sustaining them in large populations for long periods requires extensive resources and a strong public health commitment. For example, latex condoms effectively prevent sexual transmission of HIV, but making them available and educating infected persons or their sex partners to use them correctly and consistently has been accomplished only with extraordinary efforts in a few nations or settings. Some prevention efforts are considered controversial or are opposed by religious or other groups who interpret prevention efforts to reflect an acceptance of behaviors they do not condone on moral grounds.

The research effort to develop a vaccine to prevent HIV infection has been intense, but the biologic obstacles to success are immense and unprecedented. Because HIV permanently infects cells of the immune system, infection of a single cell results in lifelong infection for the host. Thus, a completely effective vaccine would need to prevent even a single cell from becoming infected. No such vaccine exists for any infection, so HIV will require a new vaccine paradigm. Possible lines of research include stimulating the immune system to detect and eliminate HIV-infected cells, or genetically transforming the HIV in an infected person so as to render it nonvirulent.

Further information on HIV and AIDS is widely available in many user-friendly and scholarly formats. The Internet is a rich source of information, with sites sponsored by public health agencies, such as the Joint United Nations Programme on HIV/AIDS (http://www.unaids.org) and the Centers for Disease Control and Prevention (http://www.cdc.gov) particularly recommended. Several texts, popular books, and scholarly journals have been devoted exclusively to AIDS public health issues and scientific research. The first of December has been designated World AIDS Day, and many governments, schools, and organizations sponsor community and educational events to coincide with that date each year.

D. PETER DROTMAN

(SEE ALSO: Behavioral Change; Condoms; Contagion; Epidemics; Prevention; Sexually Transmitted Diseases)