Acquired Immunodeficiency Syndrome

I

Introduction

Acquired Immunodeficiency Syndrome (AIDS), human viral disease that ravages the immune system, undermining the body’s ability to defend itself from infection and disease. Caused by the human immunodeficiency virus (HIV), AIDS leaves an infected person vulnerable to opportunistic infections—infection by microbes that take advantage of a weakened immune system. Such infections are usually harmless in healthy people but can prove life-threatening to people with AIDS. Although there is no cure for AIDS, new drugs are available that can prolong the life spans and improve the quality of life of infected people.

Transmission of HIV—the AIDS-causing virus—occurs most commonly as a result of sexual intercourse. HIV also can be transmitted through transfusions of HIV-contaminated blood or by using a contaminated needle or syringe to inject drugs into the bloodstream. Infection with HIV does not necessarily mean that a person has AIDS. Some people who have HIV infection may not develop any of the clinical illnesses that define the full-blown disease of AIDS for ten years or more. Physicians prefer to use the term AIDS for cases where a person has reached the final, life-threatening stage of HIV infection.

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Prevalence of AIDS

AIDS is one of the deadliest epidemics in human history. It was first identified in 1981 among homosexual men and intravenous drug users in New York and California. Shortly after its detection in the United States, evidence of AIDS epidemics grew among heterosexual men, women, and children in sub-Saharan Africa. AIDS quickly developed into a worldwide epidemic, affecting virtually every nation. The United Nations Program on HIV/AIDS (UNAIDS) estimates that the worldwide number of new cases of HIV infection peaked in the late 1990s with more than 3 million people newly infected each year. However, some regions of the world, especially Vietnam, Indonesia, and other countries in southeast Asia, continued to see an increase in the early 2000s. In addition, the number of people living with HIV or AIDS has continued to rise as the result of new drug treatments that lengthen life.

While cases of AIDS have been reported in every nation of the world, the disease affects some countries more than others. About 90 percent of all HIV-infected people live in the developing world. AIDS has struck sub-Saharan Africa particularly hard. Two-thirds of all people living with HIV infection reside in African countries south of the Sahara, where AIDS is the leading cause of death.

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In countries hardest hit, AIDS has sapped the population of young men and women who form the foundation of the labor force. Most die while in the peak of their reproductive years. Moreover, the epidemic has overwhelmed health-care systems, increased the number of orphans, and caused life expectancy rates to plummet. These problems have reached crisis proportions in parts of the world already burdened by war, political upheaval, or unrelenting poverty.

For statistics on AIDS and HIV cases in the United States and other countries, see the tables in this article.

III

Cause of AIDS

AIDS is the final stage of a chronic infection with the human immunodeficiency virus. There are two types of this virus: HIV-1, which is the primary cause of AIDS worldwide, and HIV-2, found mostly in West Africa. Inside the body HIV enters cells of the immune system, especially white blood cells known as T cells. These cells orchestrate a wide variety of disease-fighting mechanisms. Particularly vulnerable to HIV attack are specialized “helper” T cells known as CD4 cells. When HIV infects a CD4 cell, it commandeers the genetic tools within the cell to manufacture new HIV virus. The newly formed HIV virus then leaves the cell, destroying the CD4 cell in the process. No existing medical treatment can completely eradicate HIV from the body once it has infected human cells.

The loss of CD4 cells endangers health because these cells help other types of immune cells respond to invading organisms. The average healthy person has over 1,000 CD4 cells per microliter of blood. In a person infected with HIV, the virus steadily destroys CD4 cells over a period of years, diminishing the cells’ protective ability and weakening the immune system. When the density of CD4 cells drops to 200 cells per microliter of blood, the infected person becomes vulnerable to AIDS-related opportunistic infections and rare cancers, which take advantage of the weakened immune defenses to cause disease.

IV

How HIV Infection Spreads

Scientists have identified three ways that HIV infections spread: sexual intercourse with an infected person, contact with contaminated blood, and transmission from an infected mother to her child before or during birth or through breast-feeding.

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