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

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 are a type of white blood cell called CD4 cells.

HIV takes over CD4 cells and turns them into virus factories that produce thousands of viral copies. As the virus grows, it damages or kills 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 the immune system’s soldiers–the CD4 cells (T-cells). 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) because they take advantage of the body's weakened defenses.

When someone dies of AIDS, it is usually opportunistic infections or other long-term effects of HIV infection that cause death. AIDS refers to the body’s immune-compromised state that can no longer stop OIs from developing and becoming so deadly.

You don't have AIDS as soon as you are infected with HIV. You can be HIV-positive 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 develop more serious OIs.

The Centers for Disease Control and Prevention (CDC) defines someone as having AIDS if he or she is HIV-positive and meets one or both of these conditions:

• Has had at least one of 21 AIDS-defining opportunistic infections
• Has had a CD4 cell count (T-cell count) of 200 cells or less (a normal CD4 count varies by laboratory, but usually is in the 600 to 1,500 range)

Most people can not tell that they have been exposed or infected. It can take up to 12 weeks for an HIV test to come back positive. Symptoms may occur within two to four weeks of exposure to HIV, and can include flu-like symptoms such as fever, swollen glands, muscle aches, or rash.

The only way to know for sure if you are infected is take an HIV test. If you are infected, your immune system will make antibodies to fight the virus. The HIV test looks for these antibodies. If you have them in your blood, it means that you have HIV infection.

The CDC estimates that more than 25% of HIV-positive people are unaware of their HIV status. 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, or social or economic class can become infected. It is your actions (or the actions taken against you), that put you at risk.

You should be tested if:

• You have ever had vaginal, anal, or oral sex without a condom
• You have ever shared needles or syringes to inject drugs (including steroids or hormones)
• You are uncertain of your partner’s status or your partner is HIV-positive
• You are pregnant or are considering becoming pregnant
• You have ever been diagnosed with a sexually transmitted disease (STD)
• You have hepatitis C (HCV)

If you test HIV-positive 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. Being unaware of your status also makes it more likely to unknowingly pass HIV to others.

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

The most common test for HIV is the antibody test (called ELISA). It can be done on blood, saliva, or urine. According to the CDC, it is more than 99% accurate. Results are generally available within two weeks. (A rapid ELISA test that gives results in less than half an hour is also available.)

A positive result means your body has developed antibodies for HIV, so you are infected with the virus. To be completely certain, positive results are confirmed with a more sensitive test called the Western blot.

A negative result means your body has not developed antibodies and are probably not infected. To get truly accurate results, it's necessary to wait three to six months after your last possible exposure to the virus before being tested. That is because the immune system can take anywhere from three to twelve weeks to make antibodies. If you are tested in this "window period," you may get an unclear result or a false negative.

You can get tested at your doctor’s office, a clinic, the local health department facilities, or at a hospital. In addition, many states offer anonymous HIV testing. You can also purchase a kit that allows you to collect your own blood sample, send it to a lab for testing, and receive the results anonymously. (Only the "Home Access" brand kit is approved by the Food and Drug Administration.). To find a testing location near you, go to www.hivtest.org.

It is important to get tested at a site 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-positive 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.

The CDC's National AIDS Hotline can also answer questions about HIV testing and refer you to testing sites in your area:

1-800-342-2437 (English)
1-800-344-7432 (Spanish)

There is currently no vaccine available. The best way to prevent HIV is to use sterile needles and practice safer sex .