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AIDS or acquired immunodeficiency syndrome,fatal disease caused by a rapidly mutating retrovirus that attacks the immune system and leaves the victim vulnerable to infections, malignancies, and neurological disorders. It was first recognized as a disease in 1981. The virus was isolated in 1983 and was ultimately named the human immunodeficiency virus (HIV). There are two forms of the HIV virus, HIV-1 and HIV-2. The majority of cases worldwide are caused by HIV-1. In 1999 an international team of genetic scientists reported that HIV-1 can be traced to a closely related strain of virus, called simian immunodeficiency virus (SIV), that infects a subspecies of chimpanzee (Pan troglodytes troglodytes) in W central Africa. Chimpanzees are hunted for meat in this region, and it is believed the virus may have passed from the blood of chimpanzees into humans through superficial wounds, probably in the early 1930s.

Action of the Virus:

In a process still imperfectly understood, HIV infects the CD4 cells (also called T4 or T-helper cells) of the body's immune system, cells that are necessary to activate B-lymphocytes and induce the production of antibodies (see immunity). Although the body fights back, producing billions of lymphocytes daily to fight the billions of copies of the virus, the immune system is eventually overwhelmed, and the body is left vulnerable to opportunistic infections and cancers.

Signs and Symptoms:

Some people develop flulike symptoms shortly after infection, but many have no symptoms. It may be a few months or many years before serious symptoms develop in adults; symptoms usually develop within the first two years of life in infants infected in the womb or at birth. Before serious symptoms occur, an infected person may experience fever, weight loss, diarrhea, fatigue, skin rashes, shingles (see herpes zoster), thrush, or memory problems. Infants may fail to develop normally.

The definition of AIDS has been refined as more knowledge has become available. In general it refers to that period in the infection when the CD4 count goes below 200 (from a normal count of 1,000) or when the characteristic opportunistic infections and cancers appear. The conditions associated with AIDS include malignancies such as Kaposi's sarcoma, non-Hodgkin's lymphoma, primary lymphoma of the brain, and invasive carcinoma of the cervix. Opportunistic infections characteristic of or more virulent in AIDS include Pneumocystis carinii pneumonia, herpes simplex, cytomegalovirus, and diarrheal diseases caused by cryptosporidium or isospora. In addition, hepatitis C is prevalent in intravenous drug users and hemophiliacs with AIDS, and an estimated 4 to 5 million people who have tuberculosis are coinfected with HIV, each disease hastening the progression of the other. Children may experience more serious forms of common childhood ailments such as tonsillitis and conjunctivitis. These infections conspire to cause a wide range of symptoms (coughing, diarrhea, fever and night sweats, and headaches) and may lead to extreme weight loss, blindness, hallucinations, and dementia before death occurs.

Transmission and Incidence:

HIV is not transmitted by casual contact; transmission requires a direct exchange of body fluids, such as blood or blood products, breast milk, semen, or vaginal secretions, most commonly as a result of sexual activity or the sharing of needles among drug users. Such a transmission may also occur from mother to baby during pregnancy or at birth. Saliva, tears, urine, feces, and sweat do not appear to transmit the virus.

By the end of 2002 it was estimated that 42 million people were infected with HIV worldwide, the great majority in Third World countries; some 25 million had died from AIDS. The disease in sub-Saharan Africa, which has been especially hard hit, in the main has been transmitted heterosexually and has been exacerbated by civil wars and refugee problems and less restrictive local mores with regard to sex. Some 29 million people were infected with HIV in this region, where, in many countries, the prevalence of AIDS has lowered the life expectancy. The epidemic also has manifested itself in Asia (especially in India, China, Myanmar, Thailand, and Cambodia) and Latin America.

In the United States, the demographics of AIDS have changed over time. In the 1980s it was seen mainly in homosexual and bisexual men and was one of the spurs to the gay-rights movement, as activists lobbied for research and treatment monies and began education and prevention programs. Also in the early years, before careful screening of blood products was deemed necessary, the virus was contracted by an estimated 9,000 hemophiliacs (see hemophilia), and a small number of people were infected by surgical or emergency blood transfusions. Before long, however, the majority of new HIV infections were seen in drug users who contracted the disease from shared needles or unprotected sex. A large proportion of infected women are drug users or partners of drug users. Nearly a third of the infants born to HIV-infected women are infected with the virus. (Some of these infants test positive for AIDS only because of the mother's antibodies and later test negative.)

Tests and Treatment:

Various blood tests now are used to detect HIV. The most frequently used test for detecting antibodies to HIV-1 is enzyme immunoassay. If it indicates the presence of antibodies, the blood is more definitively tested with the Western blot method. A test that measures directly the viral genes in the blood is helpful in assessing the efficacy of treatments.

There is no cure for AIDS. Drugs such as AZT, ddI, and 3TC, which are reverse transcriptase inhibitors, have proved effective in delaying the onset of symptoms in certain subsets of infected individuals. The addition of a protease inhibitor, such as saquinovir, amprenavir, or atazanavir, to AZT and 3TC has proved very effective, but the drug combination does not eliminate the virus from the body. Efavirenz (Sustiva), another type of reverse transcriptase inhibitor, must be taken with protease inhibitors or older AIDS medicines. Opportunistic infections are treated with various antibiotics and antivirals, and patients with malignancies may undergo chemotherapy. These measures may prolong life or improve the quality of life, but drugs for AIDS treatment may also produce painful or debilitating side effects.

Some 30 experimental AIDS vaccines have been developed and tested, but none has yet proved clearly effective, including one that underwent full-scale testing. The development of a successful vaccine against AIDS has been slowed because HIV mutates rapidly, causing it to become unrecognizable to the immune system, and because, unlike most viruses, HIV attacks and destroys essential components of the very immune system a vaccine is designed to stimulate.

Governments and the pharmaceutical industry continue to be under pressure from AIDS activists and the public in general to find a cure for AIDS. Attempts at prevention through teaching “safe sex” (i.e., the relatively safer sex accomplished by the use of condoms), sexual abstinence in high-risk situations, and the dangers to drug users of sharing needles have been impeded by those who feel that such education gives license to promiscuity and immoral behaviors.