HIV stands for Human Immunodeficiency Virus. HIV is the virus that causes AIDS.

Your immune system is your body's defense system. While many viruses can be controlled by the immune system, HIV targets and infects the same immune system cells that are supposed to protect us from illnesses. These cells are a type of white blood cell called CD4 cells (sometimes called T-cells).

HIV takes over CD4 cells and turns them into factories that produce thousands of copies of the virus. As the virus makes copies, it damages or kills the CD4 cells, weakening the immune system.

AIDS stands for Acquired Immune Deficiency Syndrome. AIDS is the most advanced stage of HIV infection.

HIV causes AIDS by attacking CD4 cells, which the immune system uses to protect the body from disease. When the immune system loses too many CD4 cells, you are less able to fight off infection and can develop serious, often deadly, infections. These are called opportunistic infections (OIs).

When someone dies of AIDS, it is usually OIs or other long-term effects of HIV that cause death. AIDS refers to the weakened state of the body’s immune system that can no longer stop OIs.

You don't have AIDS as soon as you are infected with HIV. You can live with HIV (be HIV+) for many years with no signs of disease, or only mild-to-moderate symptoms. But without treatment, HIV will eventually wear down the immune system in most people to the point that they have low numbers of CD4 cells and develop OIs.

The Centers for Disease Control and Prevention (CDC) identifies someone as having AIDS if she or he is HIV+ and has one or both of these conditions:

• At least one AIDS-defining opportunistic infection (see our list of  AIDS Defining Conditions)
• A CD4 cell count of 200 cells or less (a normal CD4 count is about 500 to 1,500)

People with AIDS can rebuild their immune system with the help of HIV drugs just like people with HIV who do not have AIDS. Even if your CD4 cell count goes back above 200 or an OI is successfully treated, you will still have a diagnosis of AIDS. This does not necessarily mean you are sick or will get sick in the future. It is just the way the public health system counts the number of people who have had advanced HIV disease.

Most people cannot tell that they have been exposed or infected. Symptoms of HIV infection may show up within two to four weeks of exposure to HIV, and can include a fever, swollen glands, muscle aches, headache, or rash. Some people do not notice the symptoms because they are mild, or people think they have a cold or the flu. The only way to know for sure if you are infected is to take an HIV test.

The CDC estimates that about 21% of HIV+ people do not know they have HIV. Many of these people look and feel healthy and do not think they are at risk. But the truth is that anyone of any age, gender, race, sexual orientation, social group, or economic class can become infected. For more on how HIV is spread, see our info sheet on HIV transmission.

Answer the following questions:

• Have you ever had vaginal, anal, or oral sex without a condom or other latex barrier (e.g., dental dam)?
• Have you ever shared needles or syringes to inject drugs (including steroids or hormones)?
• Are you uncertain of your partner’s status or is your partner HIV+?
• Are you pregnant or considering becoming pregnant?
• Have you ever had a sexually transmitted disease (STD)?
• Do you have hepatitis C (HCV)?
  

If you answered yes to any of these questions, you should definitely get an HIV test. In the US, it is now recommended that everyone age 13-64 be screened for HIV at least once.

If you test HIV+ there are effective medications to help you stay well. But you cannot get the health care and treatment you need if you do not know your HIV status (whether you are HIV+ or HIV-negative). Being unaware of your status also means that you could pass HIV to others without knowing it.

For women who plan to become pregnant, testing is especially important. If a woman is infected with HIV, medical care and certain drugs given during pregnancy can lower the chance of passing HIV to her baby.

If you test HIV-negative you can take steps to stay that way. You can also avoid unnecessary worrying.

If you have been infected, your immune system will make antibodies against HIV. Antibodies are special proteins that our bodies make to identify “intruders” like viruses and bacteria. The most common HIV tests look for these antibodies. The HIV antibody test (called ELISA or EIA) is one of the most reliable medical tests. It can be done on blood, urine, or saliva. Some antibody tests take a few days for results, but rapid tests are also available that can give results in about 20 minutes.

A negative antibody test result means your body has not developed antibodies and you are probably not infected. To get truly accurate results, it is necessary to wait three months after your last possible exposure to the virus before being tested. This is because the immune system can take anywhere from two to eight weeks to make antibodies. If you are tested in the "window period" between infection and production of antibodies, you may get an unclear result or a false negative (testing negative when you are really HIV+).

All positive antibody tests must be followed by another test called the Western Blot to confirm the results. Results of this confirmatory test can take a few days to a few weeks. According to the CDC, the combined accuracy of the antibody test plus the Western Blot is greater than 99 percent.

There is another type of test called the RNA or PCR test, which detects the virus directly. The RNA test can give results very early in HIV infection (within weeks), but is more costly and used less often than antibody tests. This test is used for babies born to HIV+ mothers. Antibody testing on these newborns is inaccurate since they carry their mothers’ antibodies for several months after birth.

There is also a relatively new combination virus/antibody test that was approved by the Food and Drug Administration (FDA) in June 2010. Since the test detects the virus in addition to antibodies to HIV, it can be used to diagnose infection early - before antibodies develop (click here for more info).

You can get tested at your health care provider’s office, a clinic, a hospital, or facilities provided by your local health department. In addition, many states offer anonymous HIV testing and testing is available at many community-based organizations.

It is a good idea to get tested at a place that provides counseling. Counselors can answer questions about high-risk behavior and suggest ways you can protect yourself and others in the future. People who test HIV+ can receive support and referrals to health care and other services. The counselor can help you through the whole testing process – from start to finish.

To find a testing location near you, go to www.hivtest.org. The CDC's National AIDS Hotline at 1-800-CDC-INFO (232-4636) is available to answer questions about HIV testing and refer you to testing sites in your area.

You can also purchase a kit that allows you to collect your own blood sample at home, send it to a lab for testing, and receive the results anonymously. Although home HIV tests are sometimes advertised on the internet, currently only the Home Access HIV-1 Test System is approved by the Food and Drug Administration (FDA).

There is currently no vaccine available. The best way to prevent HIV is to use sterile needles and practice safer sex . (For more info, see our info sheet on AIDS Vaccines).