Menu

What Is HIV/AIDS?

AIDS (acquired immune deficiency syndrome) is a major worldwide epidemic. The disease is caused by infection with HIV (human immunodeficiency virus), which kills or harms cells of the body's immune system (T-cells), gradually destroying the body's ability to fight infections and certain cancers. There are two types of HIV, HIV-1, which is distributed worldwide, and HIV-2, which is largely confined to West Africa. People diagnosed with AIDS are likely to get life-threatening diseases called opportunistic infections which are caused by bacteria, viruses and other types of microscopic organisms that are usually harmless to healthy people. AIDS is called "acquired" to distinguish it from inherited (genetic) forms of immunodeficiency. It is called a "syndrome" because it is a set of symptoms that occur together, rather than a clear-cut disease.

How HIV Causes AIDS

An important focus of the National Institute of Allergy and Infectious Diseases (NIAID) is research devoted to the pathogenesis of human immunodeficiency virus (HIV) disease ¾ the complex mechanisms that result in the destruction of the immune system of an HIV-infected person. A detailed understanding of HIV and how it establishes infection and causes the acquired immunodeficiency syndrome (AIDS) is crucial to identifying and developing effective drugs and vaccines to fight HIV and AIDS. This fact sheet summarizes what scientists are learning about this process and provides a brief glossary of terms.

Overview

HIV disease is characterized by a gradual deterioration of immune function. Most notably, crucial immune cells called CD4+ T cells are disabled and killed during the typical course of infection. These cells, sometimes called "T-helper cells," play a central role in the immune response, signalling other cells in the immune system to perform their special functions.

A healthy, uninfected person usually has 800 to 1,200 CD4+ T cells per cubic millimeter (mm3) of blood. During HIV infection, the number of these cells in a person's blood progressively declines. When a person's CD4+ T cell count falls below 200/mm3, he or she becomes particularly vulnerable to the opportunistic infections and cancers that typify AIDS, the end stage of HIV disease. People with AIDS often suffer infections of the intestinal tract, lungs, brain, eyes and other organs, as well as debilitating weight loss, diarrhea, neurologic conditions and cancers such as Kaposi's sarcoma and lymphomas.

Most scientists think that HIV causes AIDS by directly killing CD4+ T cells or interfering with their normal function, and by triggering other events that weaken a person's immune function. For example, the network of signalling molecules that normally regulates a person's immune response is disrupted during HIV disease, impairing a person's ability to fight other infections. The HIV-mediated destruction of the lymph nodes and related immunologic organs also plays a major role in causing the immunosuppression seen in people with AIDS.

Scope of the HIV Epidemic

Although HIV was first identified in 1983, studies of previously stored blood samples indicate that the virus entered the U.S. population sometime in the late 1970s. In the United States, 612,078 cases of AIDS, and 379,258 deaths among people with AIDS had been reported to the Centers for Disease Control and Prevention (CDC) as of June 30, 1997. AIDS is now the second leading killer of people aged 25 to 44 in this country. Despite an overall stabilization in the number of new AIDS cases in this country, the epidemic continues to accelerate in certain segments of the population, notably among women and injection drug users.

Worldwide, an estimated 30.6 million people were living with HIV/AIDS as of December 1997, a figure that is projected to reach 40 million by the year 2000. More than 75 percent of all adult HIV infections have resulted from heterosexual intercourse. Through 1997, cumulative HIV/AIDS-associated deaths worldwide numbered approximately 11.7 million ¾ 9 million adults and 2.7 million children.

Treatment of AIDS and HIV Infection

Anti-HIV (also called antiretroviral) medications are used to control the reproduction of the virus and to slow or halt the progression of HIV-related disease. When used in combinations, these medications are termed Highly Active Antiretroviral Therapy (HAART). HAART combines three or more anti-HIV medications in a daily regimen, sometimes referred to as a "cocktail". Anti-HIV medications do not cure HIV infection and individuals taking these medications can still transmit HIV to others. Anti-HIV medications approved by the U.S. Food and Drug Administration (FDA) fall into four classes:

1. Nonnucleoside Reverse Transcriptase Inhibitors (NNRTIs), such as nevirappine (Viramune) and efavirenz (Sustiva), bind to and block the action of reverse transcriptase, a protein that HIV needs to reproduce.

2. Nucleoside Reverse Transcriptase Inhibitors (NRTIs), such as zidovudine (Retrovir), tenofovir DF (Viread), and stavudine (Zerit), are faulty versions of building blocks that HIV needs to make more copies of itself. When HIV uses an NRTI instead of a normal building block, reproduction of the virus is stalled.

3. Protease Inhibitors (PIs), such as lopinavir/ritonavir (Kaletra), disable protease, a protein that HIV needs reproduce itself.

4. Fusion Inhibitors, such as enfuvirtide (Fuzeon ), are newer treatments that work by blocking HIV entry into cells.

(View more complete list of HIV drugs).

How many pills you will need to take and how often you will take them depends on what medications you and your doctor choose.

There is no one "best" regimen. You and your doctor will decide which medications are right for you. For people taking HAART for the first time, the recommended regimens are:

Sustiva + Truvada, Sustiva + Epzicom, or Atripla
Kaletra + Truvada, Kaletra + Epzicom, or Kaletra + Combivir

In general, taking only one or two drugs is not recommended because any decrease in viral load is almost always temporary without three or more drugs. The exception is the recommendation for pregnant women, who may take Combivir plus nevirapine to reduce the risk of passing HIV to their infants. If you are pregnant or considering becoming pregnant, there are additional treatment considerations. Recently, a number of drugs have been developed that combine two or even three separate medications in a single pill. Some of these, such as Truvada (emtricitabine + tenofovir) and Epzicom (abacavir + lamivudine) need be taken only once daily. Atripla (emtricitabine + tenofovir + efavirenz) combines three drugs in one pill and needs to be taken only once daily, thereby providing a complete HAART regimen with one pill once daily.

The treatment of HIV infection and AIDS is in a highly dynamic state. Individuals with this condition are advised to seek out experts in their local community who are current with the latest modes of therapy and ongoing clinical trials for evaluating newer therapies.