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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.
The disease process
As HIV infection progresses, most people experience a gradual decrease in the number of cells in their blood called CD4 T cells. These cells normally protect the body from infections and other types of diseases.
Symptoms usually appear when the T-cell level drops below 200. Some people become so ill from the symptoms of AIDS that they are unable to hold a job or do household chores, while others may experience phases of intense life-threatening illness followed by periods of normal functioning. The term AIDS applies to the most advanced stages of HIV infection, and includes all HIV-infected people who have fewer than 200 CD4 T cells. (Healthy adults usually have counts of 1,000 or more).
Persistent or severe symptoms may not appear for a long time after HIV infection. However, HIV continues to actively infect and kill cells of the immune system, even when the person has no symptoms.
A few people who were infected with HIV 10 years ago or more have not yet developed symptoms. Scientists are trying to find out why the disease does not progress in these people. Possible factors include particular characteristics of their immune systems or infection with a less aggressive strain of HIV, or their genetic makeup may protect them from the effects of HIV.
How HIV spreads
HIV spreads most often by sexual contact with an infected partner. The virus enters the body through the lining of the vagina, vulva, penis, rectum or mouth during sex.
HIV also spreads through contact with infected blood. Before 1985, HIV was transmitted through transfusions of contaminated blood or blood components such as those given to people with hemophilia. Today, pre-donor screening and heat-treating techniques for blood products have practically eliminated the risk of getting HIV from transfusions.
HIV often spreads among users of intravenous (injected) drugs by sharing needles or syringes contaminated with blood from an infected individual. However, transmission by accidental needle sticks or other medical contact between patients and health care workers is extremely rare.
Women can transmit HIV to their babies during pregnancy or while giving birth. HIV also can spread to babies through the breast milk of infected mothers.
Although HIV can be found in the saliva of infected individuals, no evidence exists that the virus can spread by contact with saliva, such as by kissing. In fact, saliva contains natural compounds that reduce the ability of HIV to cause infection. There is also no evidence that HIV is spread through sweat, tears, urine or feces.
HIV is not spread through casual contact such as the sharing of food utensils, towels and bedding, swimming pools, telephones or toilet seats nor is HIV spread by being bitten by insects such as mosquitoes or bedbugs.
Who gets HIV infection?
AIDS is a worldwide epidemic without boundaries related to geography, race, age, sex or sexual orientation. Since 1981, when the U.S. Centers for Disease Control and Prevention started to monitor AIDS cases, more than 1.5 million Americans have been diagnosed with HIV. At least 988,376 Americans have been diagnosed with AIDS, and more than 500,000 have died of it.
HIV can infect anyone who uses risky behaviors such as:
- Sharing drug needles or syringes, or
- Having unprotected sexual contact with an infected person or with someone whose HIV status is unknown.
People who have another sexually transmitted disease (such as syphilis, herpes, Chlamydia or gonorrhea) are more likely than other people to get HIV during sex with an infected partner.
About one-fourth to one-third of all untreated pregnant women infected with HIV pass the infection to their babies.
This article was reviewed and updated June 2007.
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