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What is HIV?

Human immunodeficiency virus (HIV) attacks and weakens the immune system, making an individual more vulnerable to serious illness. Untreated HIV can lead to AIDS, which occurs when the immune system is so weak it becomes susceptible to serious infections and some cancers.

There’s an epidemic of HIV in the United States and around the world. According to the Centers for Disease Control and Prevention (CDC), more than 1.1 million people in the United States are living with HIV, and 1 in 7 of them aren’t aware of it. An estimated 39,782 people in the country were diagnosed with HIV in 2016 alone.

HIV transmission occurs in many different ways, including through condomless sex and by sharing needles. Risk of transmission varies depending on several factors including:

  • sexual practices and the HIV status of sexual partners
  • sharing needles for drug use or tattoos
  • use of PrEP, PEP, condoms, or having an undetectable viral load

It’s important to understand the risk level based on actual factors in preventing the transmission of HIV.

HIV can be transmitted through semen, vaginal secretions, blood, and anal secretions. When a person doesn’t use a condom during sex, it’s easier for semen, vaginal fluids, blood, and anal secretions to enter their body — either being absorbed across the mucous membrane of the vagina or anus or entering the bloodstream directly.

Anal sex is a known risk factor for contracting HIV if other prevention methods are absent, especially for the “receptive” partner whose anus is being penetrated by the penis.

Vaginal sex can also lead to HIV transmission if other prevention methods are absent, especially for the “receptive” partner whose vagina is being penetrated by the penis.

Both anal and vaginal sex can also carry a risk of HIV transmission for the “insertive” partner (that is, the person whose penis is inserted into the anus or vagina).

Oral sex (either mouth on penis or vulva/vagina) is thought to be very low risk. Rimming (mouth on partner’s anus) is also thought to very low risk.

Bottoming vs. topping

“Topping” and “bottoming” are common names for positions in anal sex. The person topping is the partner inserting their penis into the anus/rectum of their partner. The person bottoming is in the receptive position — the one whose anus/rectum is being penetrated by the other partner’s penis.

HIV can be transmitted to either partner regardless of who is topping or bottoming, especially during anal sex without a condom. Bottoming carries more risk than topping. That’s because the lining of the rectum is fragile and can tear easily during anal sex, even if blood isn’t observed and there is no pain. These microscopic tears can create a route for HIV-containing fluids, such as semen, to enter the body.

Male vs. female partners

When having vaginal sex without a condom with a partner who has a penis, the vaginal membranes are more likely to tear (even if blood isn’t visible) than the partner’s penis.

In condomless anal sex with a partner who has a penis, the rectal membranes are also more likely to tear (even if blood isn’t visible) than the partner’s penis. Microscopic tears create an easier path for HIV and other STIs to enter the body when exposed.

It’s possible for a partner with a penis to contract HIV during vaginal and anal sex. If a female partner is living with HIV with a detectable viral load, it can be carried in her vaginal secretions. If her partner has open sores on their mouth or penis, they can create a gateway for vaginal secretions or other bodily fluids with HIV to enter the body.

Uncircumcised men are at higher risk of contracting HIV from condomless sex than circumcised men. The delicate membranes of foreskin can tear during sex, creating a pathway for HIV to enter the body.

If a condom is correctly used during sex, the chances of contracting HIV and some STIs will be substantially lower. There are also different methods of protection during sexual activity including use of pre-exposure prophylaxis (PrEP), post-exposure prophylaxis (PEP), and treatment as prevention.


PrEP is a prescription daily antiretroviral medication that an HIV-negative person can take to lower their risk of contracting HIV. According to the CDC, daily PrEP reduces the risk of contracting HIV from sex by around 99 percent.

The US Preventive Services Task Force now recommends a PrEP regimen for all people at increased risk of contracting HIV.


PEP refers to taking prescription antiretroviral medications after a recent possible exposure to HIV. It’s for use in emergency situations and must be started with 72 hours of the possible exposure.

Treatment as prevention

“Treatment as prevention” refers to taking antiretroviral medication to reduce the viral load of a person living with HIV. Reducing the viral load helps a person with HIV stay healthy, and it also lowers the risk of that person transmitting HIV to a sexual partner.

When their viral load is reduced to such a low level that blood test can’t detect it (undetectable viral load), that person won’t be able to transmit HIV to a partner. An undetectable viral load virtually eliminates the risk of HIV transmission, even if the other partner isn’t on PrEP and condoms aren’t used.

Individuals with other STIs may be more likely to contract HIV.


First, some STIs like syphilis and herpes cause ulcers, or sores, to develop in the genital area or mouth. These sores create an opening in the skin, making it easier for HIV to enter the body, if exposed.

Second, when a person has an infection, their immune system sends out certain cells to help fight it. These cells are called CD4+ cells. They’re the same cells that HIV targets. When their immune system is actively fighting off another infection, they may be more susceptible to HIV.

If a partner has HIV with a detectable viral load and is also has another STI, the risk of HIV transmission increases. People with both HIV and other STIs may tend to have higher concentrations of the virus in their genital fluids. As a result, they’re more likely to transmit HIV to their sexual partner.

HIV isn’t transmitted only through sexual contact. Sharing needles also puts a person at higher risk of contracting HIV.

When a needle is injected into a person’s body, it breaks the skin barrier. If the needle has already been injected into another person, it can carry traces of their blood, along with any infections they have. The contaminated needle can introduce these infections into the second person’s body.

Researchers don’t know if having an undetectable viral load reduces the risk of HIV transmission through shared needles, but it’s reasonable to assume it may provide some risk reduction.

HIV can affect anyone. Whatever their age, gender, sexuality, ethnicity, or race, everyone should take steps to protect themselves. But due to socioeconomic factors, some demographic groups have higher HIV transmission rates and generally are more affected by HIV.

According to the CDC, the general demographic traits most affected by HIV are:

  • Age and location. In 2016, 37 percent of people newly diagnosed with HIV in the United States were between the ages of 20 and 29, while another 25 percent were aged 30 to 39. The South had the highest number of new diagnoses in 2016.
  • Sexuality and race. Men who have sex with men is the population most affected by HIV. In 2016, this group accounted for 67 percent of all new HIV diagnoses, and 83 percent of new diagnoses among males. African American men in this group have the highest diagnoses of any specific population.
  • Ethnicity. African Americans comprised only 12 percent of the American population in 2016, but they accounted for roughly 44 percent of new HIV diagnoses. Hispanics and Latinos represented 18 percent of the population in 2016 but accounted for 25 percent of new HIV diagnoses.

Transgender women are also highly impacted by HIV transmissions as a population, reports the CDC.

These groups are disproportionately affected by HIV, but they aren’t inherently at greater risk of contracting HIV. An individual’s personal risk depends on their behaviors, not on their age, gender, sexuality, ethnicity, race, or any other demographic factor.

To lower the risk of getting HIV and other STIs:

  • Those who are HIV-negative should consider PrEP. If a possible HIV exposure occurs, PEP may provide emergency protection.
  • Use condoms during vaginal and anal sex.
  • Get tested and treated for STIs and follow healthcare providers’ recommended screening schedule.
  • Before having sex with someone, ask them to get tested for HIV and STIs.
  • Those who inject drugs should get clean needles from a needle exchange.
  • Avoid sharing needles for drugs and tattoos.

Talk to a healthcare provider about PrEP if a sexual partner has HIV with a detectable viral load or there’s another known risk of contracting the virus. Here’s a search tool for finding healthcare providers who prescribe PrEP.

Anyone who thinks they might have contracted HIV needs to get tested immediately. Early treatment can help manage the symptoms, lower the risk of complications, lower the risk of transmitting HIV to a sexual partner, and help people to live a long and healthy life.

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Read this article in Spanish.