HIV (human immunodeficiency virus) is the virus that causes AIDS. Find out more facts about HIV infection.

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AIDS (acquired immune deficiency syndrome) is caused by infection with HIV (human immunodeficiency virus). Without treatment, HIV kills or harms cells of the body's immune system. Over time, this destroys the body's ability to fight infections and certain cancers.

People who have AIDS are likely to get life-threatening diseases called opportunistic infections. These are caused by bacteria, viruses, and other types of microscopic organisms that are usually harmless to healthy people.

AIDS is called acquired because it's not an inherited (genetic) form 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 have a gradual decrease in the number of cells in their blood called CD4 T cells or T-helper cells. These cells normally protect the body from infections and other types of diseases. Healthy adults usually have CD4 T counts of 500 to 1,500.

Symptoms usually appear when the CD4 T-cell level drops below 200. Some people become so ill from the symptoms of AIDS that they are unable to work or do household chores. Others may have phases of intense life-threatening illness followed by periods of normal functioning.

Persistent or severe symptoms may not appear for a long time after HIV infection. Yet, HIV can continue to actively infect and kill cells of the immune system, even when the person has no symptoms. The diagnosis of AIDS is made when:

  • Your CD4 T cell count drops below 200, or
  • You develop an opportunistic infection

Some 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 may include certain characteristics of their immune systems or infection with a less aggressive strain of HIV. Or it may be something in their genetic makeup that protects 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 sexual activity.

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 all but eliminated the risk of getting HIV from blood transfusions.

HIV often spreads among users of intravenous (IV or injected) drugs through 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.

There have been no proven cases of HIV being transmitted through saliva. But there is a risk of HIV with oral sex or deep kissing, especially if you have bleeding gums or mouth sores.

HIV is not spread through casual contact such as the sharing of food utensils, towels and bedding, swimming pools, telephones, or toilet seats. It's also not spread by insect bites, such as from mosquitoes or bedbugs.

Who gets HIV infection?
HIV can infect anyone who practices risky behaviors, such as:

  • Sharing drug needles or syringes
  • 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 or herpes, 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.

By Louis Neipris, MD, Staff Writer
Created on 07/28/1999
Updated on 12/28/2010
  • U.S. Department of Health and Human Services. AIDS.info. Testing HIV positive - do I have AIDS?
  • AIDS.gov. What is HIV/AIDS?
  • Centers for Disease Control and Prevention. HIV/AIDS. Human immunodeficiency virus type 2.
  • AIDSInfoNet. What is AIDS?
  • Centers for Disease Control and Prevention. HIV/AIDS. Basic statistics.
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