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Basic Questions and Answers About HIV Transmission

January 16, 2015

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Basic Questions and Answers About HIV Transmission

Myths persist about how HIV is transmitted. This section provides the facts about HIV risk from different types of sex, injection drug use, and other activities.

Table of Contents

How Is HIV Passed From One Person to Another?

In the United States, HIV is spread mainly by having sex or sharing injection drug equipment such as needles with someone who has HIV.

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 sex with someone who has HIV. In general:
    • 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 health care 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.


How Well Does HIV Survive Outside the Body?

HIV does not survive long outside the human body (such as on surfaces), and it cannot reproduce. It is not spread by

  • Air or water.
  • Insects, including mosquitoes or ticks.
  • Saliva, tears, or sweat. There is no documented case of HIV being transmitted by spitting.
  • Casual contact like shaking hands or sharing dishes.
  • Closed-mouth or "social" kissing
  • Toilet seats.

Can I Get HIV From Anal Sex?

Yes. In fact, having anal sex is the riskiest type of sex for getting or spreading HIV.

HIV can be found in the blood, semen (cum), preseminal fluid (pre-cum), or rectal fluid of a person infected with the virus. The bottom is at greater risk of getting HIV because the lining of the rectum is thin and may allow HIV to enter the body during anal sex, but the top is also at risk because HIV can enter through the opening of the penis or through small cuts, abrasions, or open sores on the penis. See the Prevention Q&As for more information.

Can I Get HIV From Vaginal Sex?

Yes. In general vaginal sex is not as risky anal sex, but is still a high-risk behavior for HIV infection.

It is possible for either partner to become infected this way. This risk depends on many factors, including whether the partners are using condoms, whether the partner with HIV is using antiretroviral therapy (ART) consistently and correctly and whether the partner who is HIV-negative is using pre-exposure prophylaxis (PrEP) consistently and correctly. Condoms and HIV medicines can greatly lower the risk of transmitting HIV.

In women, HIV can be directly absorbed through the mucous membranes that line the vagina and cervix. The lining of the vagina can also sometimes tear and possibly allow HIV to enter the body.

In men, HIV can enter the body through the urethra (the opening at the tip of the penis) or through small cuts or open sores on the penis. Men who are not circumcised are at greater risk of HIV infection through vaginal sex than are circumcised men.

Risk for HIV infection increases if you or a partner also has a sexually transmitted disease (STD). See also Is there a connection between HIV and other sexually transmitted infections?

Many barrier methods that women use to prevent pregnancy (e.g., diaphragm, cervical cap) do not protect them against HIV or other STDs because they still allow infected semen (cum) to come in contact with the lining of the vagina.

Oral or hormonal contraceptives (e.g., birth control pills) do not protect women against HIV or other STDs.

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This article was provided by U.S. Centers for Disease Control and Prevention. Visit the CDC's website to find out more about their activities, publications and services.