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Human immunodeficiency virus (HIV) is a virus that attacks the body’s immune system. In the advanced stages, it causes acquired immunodeficiency syndrome (AIDS). HIV/AIDS is a major global public health issue. There are nearly 40 million people living with HIV and AIDS worldwide as of 2021, mainly in Africa. There is no cure for HIV infection currently, but treatment can prevent it from progressing and spreading to others.

Overview of HIV Transmission

HIV is usually transmitted through contact with blood or body fluids from a person who has HIV with a detectable viral load. The viral load is the amount of virus present in the blood of a person with HIV.

In order to transmit HIV, bodily fluids (blood, semen, rectal fluids, vaginal fluids, or breast milk) from a person infected with HIV must enter the bloodstream of a person who does not have HIV. 

HIV transmission can occur through mucous membranes (the lining of the mouth, vagina, tip of the penis, or rectum), through open sores or cuts in the skin, or through direct injection into the bloodstream with a needle.

What Are The 4 Ways HIV Can Be Transmitted?

The four most common ways HIV is transmitted are:

• Having unprotected anal or vaginal sex.

• Sharing contaminated needles or other drug injection equipment.

• Contaminated blood transfusions.

• From mother to baby during childbirth or breastfeeding.

Blood Transmission

Shared Injection Drug Equipment

Injection drug equipment, such as needles and syringes that are used by someone who has HIV can pass HIV to others who also use the equipment through contaminated blood. In addition to illicit drugs, people who inject hormones, steroids, or silicone can acquire HIV through shared injection equipment. 

Healthcare Exposure

Healthcare workers have a small risk of being exposed to HIV through needlesticks or sharps injuries. 

Blood Transfusions and Organ Transplants

Blood transmission of HIV can occur during a blood transfusion or organ/tissue transplant from an HIV-positive donor. However, this risk is extremely small because, in the U.S., all donated blood and organs/tissues are rigorously tested for HIV. In addition, the risk of possible HIV exposure while donating blood is also very low because of safe collection practices.

Rare Examples of Blood Transmission of HIV

Very rarely, HIV spreads through a bite from a person who has HIV. This can result in transmission of the virus through broken skin or wounds or contact of body fluids of the person who has HIV with the mucous membranes of the person who does not have HIV infection. However, transmission through bites requires severe trauma and the presence of blood in the wound. There is no risk of transmitting HIV infection if the skin is not broken by the bite. Also, HIV is not transmitted through saliva. 

Blood transmission of HIV can very rarely occur through deep, open-mouth kissing if both individuals have bleeding gums or mouth sores. This transmission occurs through contact with the blood of the HIV-positive person and not their saliva.

Anal or Vaginal Sex

Transmission of HIV can take place through vaginal or anal sex with a person who has HIV if you don’t correctly use a condom every time or if you and your partner are not taking antiretroviral drugs to prevent or treat HIV. In terms of specific sexual practices, anal sex has a higher risk of transmitting HIV compared to vaginal sex. 

Can You Get HIV Through Oral Sex?

The risk of HIV transmission through oral sex is very low. While it is lower than through vaginal or anal sex, it can occur if an HIV-positive person ejaculates into their partner’s mouth during oral sex and the receiving partner has breaks in the mucous membrane (oral ulcers or bleeding gums). 

Also, the presence of another sexually transmitted infection (STI), which may or may not cause visible signs such as genital sores, can increase the risk of transmitting the HIV infection. Nonetheless, the risk of HIV transmission through oral sex is much lower compared to anal or vaginal sex.

Can You Get HIV Through Sex Toys?

HIV transmission through shared sex toys, such as vibrators, dildos, and anal plugs, is rare but not impossible.

Perinatal Transmission from Mother to Child

An HIV-positive mother can transmit HIV to her baby during pregnancy, childbirth, or breastfeeding. However, the availability of effective HIV medicines and other preventive strategies have lowered the risk of mother-to-child HIV transmission to under 1% in the U.S.

Body Fluids That Transmit HIV Infection

Blood

HIV transmission through infected blood and blood products occurs when the person with HIV has a detectable viral load. Their blood enters another person's bloodstream through injection or contact with a mucous membrane (mucous membranes are parts of the body with wet, absorbent skin, such as the mouth, vagina, tip of the penis, or rectum).

Semen

Semen of an HIV+ individual contains a high concentration of HIV, second only to blood. Both semen (cum) which is secreted from the penis during ejaculation, and pre-seminal fluid (pre-cum) which is secreted before ejaculation can transmit HIV. These fluids can get into the bloodstream of the HIV-negative partner during vaginal or anal sex through the mucous membranes of the vagina or rectum.

Vaginal Fluids

Sexual transmission of HIV can occur through the vaginal fluids of HIV+ individuals, although the virus is present in smaller concentrations in vaginal fluid compared to blood and semen.

Rectal Fluids

HIV of an individual who tests positive is present in rectal fluids, and anal sex is one of the riskiest types of sex to transmit HIV.

Breast Milk

HIV is present in the breast milk of HIV+ mothers, and transmission of the infection from mother to baby can occur through breastfeeding. For mothers taking antiretroviral therapy (HIV medicines) with a sustained undetectable viral load during and after the pregnancy, the risk of transmitting an HIV infection to their baby is 1%.

Factors Affecting HIV Transmission

HIV Viral Load

Viral load is a measure of the amount of virus in the blood of a person who has HIV. Having a high viral load is a risk factor for transmitting HIV. In contrast, an undetectable viral load makes it much less likely that the person will transmit HIV to others.

• A high HIV viral load is more than 100,000 copies of the virus per milliliter (mL) of blood. This means that the virus is rapidly making copies of itself, attacking the immune system and causing disease progression.

• A low HIV viral load is less than 10,000 copies of the virus per mL of blood. This means that the virus isn’t actively multiplying to damage the immune system. 

• A detectable HIV viral load is having more than 200 copies of the virus per milliliter of blood.

• An undetectable viral load is when the amount of virus in the blood is too little to be detected on testing (less than 50 copies per mL of blood). 

Acute and Late Stage HIV

In the acute stage of the HIV infection (usually 2-4 weeks after infection), the virus multiplies fast and spreads throughout the person’s body, attacking and destroying CD4 white blood cells. The viral load during the acute HIV infection stage is very high, and this significantly increases the risk of transmitting HIV to others.

Late-stage HIV is called AIDS (acquired immunodeficiency syndrome). It is the last and most severe stage of the infection. People are diagnosed with AIDS if their CD4 white blood cell count is very low and they have certain opportunistic infections. People with AIDS have a very high viral load, which increases the risk of transmitting HIV to others. 

Other Sexually Transmitted Infections (STIs)

Sexually transmitted infections (STIs) are infections that spread through sexual activity, including oral, vaginal, or anal sex. Examples of STIs include HIV, chlamydia, gonorrhea, syphilis, and HPV. A person with a sexually transmitted infection may or may not have any symptoms. Sexually transmitted diseases (STDs) are conditions that develop if STIs are not treated. 

Having an STI or STD can make it easier to get HIV. For instance, an STD may cause a genital sore or a break in the skin, which can make it easier for HIV to enter your body. In a person who is HIV positive, having another STD increases the risk of giving HIV to others.

Gender Differences in HIV Transmission

As mentioned, anal sex carries the highest risk for HIV transmission. Consequently, the population group most affected by HIV in the United States is gay and bisexual men who have sex with men. According to the Centers for Disease Control and Prevention (CDC), over 70% of new HIV diagnoses in 2021 were reported from male-to-male sexual contact. 

Among women, heterosexual HIV transmission is the main mode of getting HIV-infected, accounting for over 80% of HIV infections in women. Receptive female partners with untreated HIV-infected insertive male partners are at the greatest risk of contracting HIV during sexual intercourse. This is because of the large viral loads present in pre-seminal and seminal fluids. Transmission of HIV from female to male during heterosexual intercourse is less likely, with about half the transmission rate compared to male-to-female transmission.

Read more about HIV Symptoms in Men: Signs to Look For.

Sexual Practices

Sexual practices can affect your risk of getting HIV if you have sex with someone who has HIV and is not taking HIV medicines. The risk of HIV infection is lowest to highest as follows:

• Closed-mouth and open-mouth kissing, touching, and oral sex: None to low

• Insertive penile-vaginal sex: Low to medium

• Receptive penile-vaginal sex: Medium

• Insertive anal sex: High

• Receptive anal sex: Highest

How Is HIV Not Transmitted?

Some of the ways in which HIV infection is not transmitted include:

• Contact with saliva, tears, or sweat.

• Breathing the same air.

• Casual contact, such as touching, shaking hands, or hugging.

• Sharing utensils.

• Consuming food handled by someone who has HIV.

• Using the same toilet seat.

• Swimming in the same pool.

• Bites from insects or other animals.

There are rare reports of infants contracting HIV by eating pre-chewed food that contains blood from a person with HIV.

Can You Get HIV From Living With Someone Who Has It?

You cannot get HIV from living with someone who has it if you only have non-sexual contact with them. People with HIV do not transmit HIV to others with whom they live at home or in the community. 

How Long Can an HIV-Positive Person Live?

Many people with HIV live as long as their peers who do not have HIV. Provided they are diagnosed and treated in good time and adhere to their HIV treatment, they can live long and healthy lives. However, if HIV is left untreated, it can progress to late-stage disease (AIDS). After the onset of AIDS, without access to antiretroviral therapy, life expectancy is 2-3 years. Death usually occurs from HIV-related health complications. 

Is There a Treatment to Cure HIV?

As mentioned, there is currently no cure for HIV. However, there is antiretroviral therapy (ART). ART works by drastically slowing down replication of the virus, especially in combination with other ART medications. ART is the standard treatment for anyone who has been diagnosed with HIV, and the therapy can significantly reduce viral load over time.

Strategies for Preventing HIV 

Safe Sexual Practices

Condoms (both male condoms and female condoms) are highly effective in preventing the transmission of HIV and other sexually transmitted infections. You should learn to use a condom correctly. For example, use only a water-based lubricant with condoms as oil-based lubricants can weaken them. Use a condom every time you have sex to prevent HIV infection, and make sure you use the correct size. 

If you are having sexual intercourse with a person who is HIV-positive or of unknown HIV status, keep in mind that receptive anal sex is the riskiest type of sex for acquiring HIV. HIV infection transmission can occur in either partner—the one inserting their penis or the one receiving the penis in their anus—but it is riskier if the HIV-negative partner is the receiver. 

Voluntary medical male circumcision can lower a male’s risk of contracting HIV by up to 60% through heterosexual contact with female partners who are HIV positive.

Other preventive measures to reduce the risk of HIV transmission if you are sexually active include:

• Encourage an HIV-positive partner to get and stay on HIV treatment. 

• Get tested for STDs at least once a year and encourage your partner to do the same. Knowing your and your partners’ HIV status can help you make good decisions and lower the risk of transmitting HIV to others.

• Reduce the number of sexual partners you have to lower your HIV risk. Having multiple sex partners is a risk factor for HIV.

Pre-Exposure Prophylaxis (PrEP) and Post-Exposure Prophylaxis (PEP)

Pre-exposure prophylaxis (PrEP) is an HIV medicine that people at risk for HIV can take to prevent getting HIV through sex. It is very effective if the medicine is taken as prescribed. PrEP may be recommended for you, for example, if you do not have HIV but your sexual partner has HIV or if you inject drugs and share needles with others. 

Post-exposure prophylaxis (PEP) is an HIV medicine that, if taken within 72 hours of possible exposure, can prevent HIV. PEP may be recommended for you, for example, if you are HIV-negative or have unknown HIV status and may have been exposed to HIV during sex (for instance, through the breakage of a condom or sexual assault). You need to take PEP for 28 days.

Needle-Exchange Programs

HIV and other blood-borne infections like hepatitis can spread through needles, syringes, and other injection materials that were previously used by someone who has HIV. The best way to prevent HIV transmission is to stop injecting drugs. However, if you are unable to stop injection drug use, using sterile injection equipment each time can lower your risk of getting HIV. Studies have shown that syringe services programs and needle exchange programs can lower the risk of transmission of HIV by more than 60-70%.

Measures During Pregnancy And Childbirth

Some of the measures that can lower the risk of HIV transmission from mother to child are:

• Get an HIV test as soon as possible to know your HIV status if you are planning a pregnancy or are pregnant (get tested early with each pregnancy).

• If your partner is HIV-positive, encourage them to stay on their HIV medicines. Talk to your healthcare provider about taking pre-exposure prophylaxis (PrEP).

• If you are HIV-positive, take your HIV medicines to achieve a suppressed viral load. Undetectable viral loads during and after pregnancy greatly reduce the risk of transmission from mother to baby.

• After delivery, talk to your healthcare provider about the risks of transmitting HIV if you breastfeed. If you have an undetectable viral load, the risk of HIV transmission through breastfeeding is low (about 1%). Consider alternative milk formulations to completely eliminate the risk of transmission to the baby during feeding.