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HIV/AIDS Basics

CDC offers basic HIV and AIDS information for both healthcare providers and the public, some of which is excerpted here. For more information, visit CDC’s HIV Basics section.

What is HIV?

HIV stands for human immunodeficiency virus. It is the virus that can lead to acquired immunodeficiency syndrome, or AIDS. Unlike some other viruses, the human body cannot get rid of HIV. That means that once you have HIV, you have it for life.

No safe and effective cure currently exists, but scientists are working hard to find one, and remain hopeful. Meanwhile, with proper medical care, HIV can be controlled. Treatment for HIV is often called antiretroviral therapy or ART. It can dramatically prolong the lives of many people infected with HIV and lower their chance of infecting others. Before the introduction of ART in the mid-1990s, people with HIV could progress to AIDS in just a few years. Today, someone diagnosed with HIV and treated before the disease is far advanced can have a nearly normal life expectancy.

HIV affects specific cells of the immune system, called CD4 cells, or T cells. Over time, HIV can destroy so many of these cells that the body can’t fight off infections and disease. When this happens, HIV infection leads to AIDS.

AIDS (acquired immunodeficiency syndrome): This is the stage of infection that occurs when your immune system is badly damaged and you become vulnerable to infections and infection-related cancers called opportunistic illnesses. When the number of your CD4 cells falls below 200 cells per cubic millimeter of blood (200 cells/mm3), you are considered to have progressed to AIDS. (Normal CD4 counts are between 500 and 1,600 cells/mm3.) You can also be diagnosed with AIDS if you develop one or more opportunistic illnesses, regardless of your CD4 count. Without treatment, people who are diagnosed with AIDS typically survive about 3 years. Once someone has a dangerous opportunistic illness, life expectancy without treatment falls to about 1 year. People with AIDS need medical treatment to prevent death.

How can I tell if I’m infected with HIV?

The only way to know if you are infected with HIV is to be tested. You cannot rely on symptoms to know whether you have HIV. Many people who are infected with HIV do not have any symptoms at all for 10 years or more.

Should I get tested for HIV?

CDC recommends that healthcare providers test everyone between the ages of 13 and 64 at least once as part of routine healthcare. One in six people in the United States who have HIV do not know they are infected.

HIV is spread through unprotected sex and drug-injecting behaviors (plus mother to child), so people who engage in these behaviors should get tested more often. If you answer yes to any of the following questions, you should definitely get an HIV test:

  • Have you had unprotected sex (sex without a condom)—anal, vaginal, or oral—with men who have sex with men or with multiple partners since your last HIV test?
  • Have you injected drugs (including steroids, hormones, or silicone) and shared equipment (or works, such as needles and syringes) with others?
  • Have you exchanged sex for drugs or money?
  • Have you been diagnosed with or sought treatment for a sexually transmitted infection (STI), like syphilis?
  • Have you been diagnosed with or sought treatment for hepatitis or tuberculosis (TB)?
  • Have you had unprotected sex with someone who could answer yes to any of the above questions or someone whose history you don’t know?

If you continue having unsafe sex or sharing injection drug equipment, you should get tested at least once a year. Sexually active gay and bisexual men may benefit from more frequent testing (e.g., every 3 to 6 months).

You should also get tested if:

  • You have been sexually assaulted.
  • You are a woman who is planning to get pregnant or who is pregnant.

How can testing help me?

Getting tested can give you some important information and can help keep you—and others—safe. For example:

  • Knowing your HIV status can give you peace of mind—and testing is the only way you can know your HIV status for sure.
  • When you and your partner know each other’s HIV status, you can make informed decisions about your sexual behaviors and how to stay safe.
  • If you are pregnant, or planning to get pregnant, knowing your status can help protect your baby from becoming infected.
  • If you find out you are HIV-positive, you can start taking medicine for your HIV. Getting treated for HIV improves your health, prolongs your life, and greatly lowers your chance of spreading HIV to others.
  • If you know you are HIV-positive, you can take steps to protect your sex partners from becoming infected.

How is HIV passed from one person to another?

Only certain fluids—blood, semen (cum), pre-seminal fluid (pre-cum), rectal fluids, vaginal fluids, and breast milk—from an HIV-infected person can transmit HIV. These fluids must come in contact with a mucous membrane or damaged tissue or be directly injected into the bloodstream (from a needle or syringe) for transmission to possibly occur. Mucous membranes can be found inside the rectum, the vagina, the opening of the penis, and the mouth.

In the United States, HIV is spread mainly by:

  • Having unprotected sex (sex without a condom) with someone who has HIV.
    • Anal sex is the highest-risk sexual behavior. Receptive anal sex (bottoming) is riskier than insertive anal sex (topping).
    • Vaginal sex is the second highest-risk sexual behavior.
    • Having multiple sex partners or having other sexually transmitted infections can increase the risk of infection through sex.
  • Sharing needles, syringes, rinse water, or other equipment (works) used to prepare injection drugs with someone who has HIV.

Less commonly, HIV may be spread by:

  • Being born to an infected mother. HIV can be passed from mother to child during pregnancy, birth, or breastfeeding.
  • Being stuck with an HIV-contaminated needle or other sharp object. This is a risk mainly for healthcare workers.
  • Receiving blood transfusions, blood products, or organ/tissue transplants that are contaminated with HIV. This risk is extremely small because of rigorous testing of the US blood supply and donated organs and tissues.
  • Eating food that has been pre-chewed by an HIV-infected person. The contamination occurs when infected blood from a caregiver’s mouth mixes with food while chewing, and is very rare.
  • Being bitten by a person with HIV. Each of the very small number of documented cases has involved severe trauma with extensive tissue damage and the presence of blood. There is no risk of transmission if the skin is not broken.
  • Oral sex—using the mouth to stimulate the penis, vagina, or anus (fellatio, cunnilingus, and rimming). Giving fellatio (mouth to penis oral sex) and having the person ejaculate (cum) in your mouth is riskier than other types of oral sex.
  • Contact between broken skin, wounds, or mucous membranes and HIV-infected blood or blood-contaminated body fluids. These reports have also been extremely rare.
  • Deep, open-mouth kissing if the person with HIV has sores or bleeding gums and blood is exchanged. HIV is not spread through saliva. Transmission through kissing alone is extremely rare.

Is the risk of HIV different for different people?

Although HIV risk factors and routes of transmission apply to everyone equally, some people are at higher risk because of where they live and who their sex partners are.

The percentage of people living with HIV (prevalence) is higher in major metropolitan areas, so people who live there are more likely to encounter an HIV-positive person among their possible sex partners. (This atlas shows the prevalence of HIV for all US communities.) In the same way, because the prevalence of HIV is higher among gay and bisexual men and among black and Latino men and women, members of these groups are more likely to encounter partners who are living with HIV.

How can I prevent getting HIV?

In the United States, HIV is spread mainly by having anal or vaginal sex without a condom or by sharing drug-use equipment with an infected person. Substance use can contribute to these risks indirectly because alcohol and other drugs can lower people’s inhibitions and make them less likely to use condoms. Today, more tools than ever are available to prevent HIV. In addition to limiting your number of sexual partners, never sharing needles, and using condoms correctly and consistently, you may be able to take advantage of newer biomedical options such as pre-exposure and post-exposure prophylaxis.

Is there a cure for HIV?

For most people, the answer is no. Most reports of a cure involve HIV-infected people who needed treatment for a cancer that would have killed them otherwise. But these treatments are very risky, even life-threatening, and are used only when the HIV-infected people would have died without them. Antiretroviral therapy (ART), however, can dramatically prolong the lives of many people infected with HIV and lower their chance of infecting others. It is important that people get tested for HIV and know that they are infected early so that medical care and treatment have the greatest effect.

Last Updated: 
5/14/15