Human immunodeficiency virus (HIV) is the cause of HIV infection. HIV infects a specific type of immune system cells called CD4 lymphocytes or helper T cells. The role of healthy CD4 cells is to organize other immune cells to respond to infection, effectively stimulating the immune response. When HIV infects a CD4 cell, it hijacks the process by which the cell replicates itself and uses it to produce copies of HIV instead, releasing them into the body to infect more CD4 cells.

HIV also destroys CD4 cells, making it difficult for the body to coordinate an immune response to invading pathogens such as viruses or bacteria. Lowered levels of CD4 cells weaken the immune system and leave the body unable to protect itself from cancer and infections.

DNA and RNA are different types of molecules used to communicate genetic instructions. Most viruses translate DNA into RNA, which instructs the cell on which proteins to produce. HIV is known as a retrovirus due to its ability to reverse the usual process, translating its RNA into DNA and inserting it into the nucleus of the cell, which then reproduces more HIV.

How Do People Become Infected With HIV?

In people infected with HIV, the virus is carried in blood, semen, pre-seminal fluid, vaginal secretions, rectal fluid, and breast milk. In order to spread HIV, the infected fluid must come into contact with the uninfected person in one of three specific ways:

• Injected directly into the bloodstream
• Contact with mucous membranes, which line the inside of the mouth, vagina, anus, and penis
• Damaged tissue such as an open wound, sore, cut, or tear — even if the damage is invisible to the naked eye

HIV is most commonly passed to another person during unprotected (without condoms) anal or vaginal sex or by sharing used needles, syringes, or other injection equipment. HIV can also be spread by contaminated blood transfusions or tissue transplants and improperly sanitized medical, and dental or tattoo instruments. HIV transmission (spread) is rare via blood transfusions, transplants, and medical instruments in countries with strict biosafety laws. Pregnant people and new mothers can also pass HIV to the fetus during pregnancy, birth, and while breastfeeding. It is extremely rare but possible to contract HIV from oral sex or deep kissing if there are open wounds — for instance, bleeding gums.

Preventing HIV Transmission

Using a new synthetic (latex or polyurethane) condom every time you have vaginal or anal sex is extremely effective for preventing HIV transmission. To be effective, the condom must be put on correctly before penetration begins. Note that “natural” condoms, such as those made from the intestinal membranes of lambs, do not prevent the transmission of HIV.

Pre-Exposure Prophylaxis

The U.S. Food and Drug Administration (FDA) has approved three medications for HIV prevention. Known as pre-exposure prophylaxis (PrEP), these medications stop HIV from replicating in your body. PrEP pills contain the drugs tenofovir and emtricitabine — sold under the brand names Truvada and Descovy. These formulations each have a different type of tenofovir, and your doctor will help you figure out which is best for you. You need to take these medications consistently every day for them to be effective.

Cabotegravir (Apretude) is a long-acting PrEP injection that’s given once every two months. Like PrEP pills, you’ll need to take the injections consistently and on time. The Centers for Disease Control and Prevention (CDC) states that, as of 2022, cabotegravir isn’t recommended for people who use injected drugs, just for preventing sexual transmission of the virus. Researchers have found that Apretude reduces transmission risk by 90 percent compared to Truvada in studies that show the most efficacy, according to the FDA.

When a person who is HIV-negative takes Truvada or Descovy properly, their risk of contracting HIV with an infected sexual partner is reduced by about 99 percent, according to the CDC. Truvada reduces the risk of contracting HIV from sharing needles with an infected person by at least 74 percent, according to the agency.

After being on treatment for several months, many people living with HIV can bring their viral load (the measure of HIV virus in their blood) down to such low levels that it is undetectable. People with an undetectable viral load are believed to be incapable of spreading HIV to others through sexual means as long as they continue to consistently take treatment. Treatment for HIV is now seen as prevention from getting HIV.

If you’re interested in learning more about PrEP to lower your risk of HIV, talk to your health care provider.

Post-Exposure Prophylaxis

If you think you may have been exposed to HIV, you can take post-exposure prophylaxis (PEP) to prevent HIV infection from taking hold. PEP contains a few specific medications that are normally used to treat HIV. PEP is most effective within 72 hours or three days of exposure to the virus. You’ll need to take your PEP medications every day for four weeks (28 days), and you should follow up with your doctor.

Unfortunately, PEP isn’t 100-percent effective. The sooner you start taking your PEP medications, the more time it has to start working. Some studies suggest that PEP lowers the risk of HIV infection by more than 80 percent. You’ll have better chances of preventing HIV if you take your PrEP medications every day as prescribed, in addition to using other prevention measures.

It’s important to talk to your primary care provider or an emergency room doctor as soon as you think you’ve been exposed. They can determine whether you need PEP and which medications to prescribe.

Types of Contact That Cannot Transmit HIV

Decades ago, when little was known about HIV and AIDS, people were not sure exactly how the virus was transmitted. Myths surrounding HIV persist, and some people still fear close contact with those who are living with HIV. However, clinical studies have proved that HIV cannot survive outside the body — including on skin, in water, or in the air — and many bodily fluids do not carry HIV.

The following types of contact cannot transmit HIV between people:

• Shaking hands
• Hugging
• Most kissing
• Sharing food or cooking utensils
• Coughing or sneezing
• Being bitten by mosquitoes or other insects
• Coming into contact with animals
• Bathing, showering, or swimming together

Additionally, it can’t be transmitted by coming into contact with another person’s:

• Tears
• Urine
• Feces
• Saliva

Risk Factors for HIV

Anyone — regardless of age, sex, sexual orientation, or ethnic background — can become infected with HIV.

Behavioral Factors

Lifestyle choices and other behavioral factors have the most influence over who gets HIV.

High-risk behaviors include:

• Having anal sex without using condoms
• Having vaginal sex without using condoms
• Having sex with multiple partners without using condoms
• Using intravenous (injected) drugs and sharing needles and other injection equipment

During sex, the person receiving penetration (the “bottom”) has the highest risk of contracting HIV. The person performing penetration (the “top”) has the highest risk of transmitting HIV.

The risk of contracting HIV increases if you have other sexually transmitted infections (STIs) such as herpes. This is because the same behaviors that raise your risk of an STI can also contribute to your risk of HIV infection. Additionally, HIV can enter the body through a skin break or open sore caused by an STI.

A male with an uncircumcised penis is more likely to contract HIV than one who is circumcised, according to the CDC. Studies have shown that circumcised males are 50 percent to 60 percent less likely to contract HIV during sex with a female than those who are uncircumcised, per the CDC.

Demographic Factors

Sexual orientation, sex, ethnicity, and geographic area all influence the risk of contracting HIV.

Men who have sex with men (MSM) have the highest rate of HIV infection, making up 70 percent of those diagnosed in the U.S. in 2021, according to HIV.gov.

Ethnic background has a strong influence over HIV risk. In the U.S., Black/African American people accounted for 40 percent of HIV diagnoses in 2021, according to HIV.gov, although they make up only 12 percent of the population. Hispanic/Latino people also have a higher risk of contracting HIV, according to HIV.gov. In 2021, around 29 percent of HIV diagnoses were in Hispanic/Latino people, although they were 18 percent of the U.S. population.

In general, people assigned male at birth (AMAB) are more likely to contract HIV than those assigned female at birth, according to HIV.gov. Data for 2021 show that 81 percent of new HIV infections occurred in people AMAB. Additionally, transgender people accounted for 2 percent of new infections. Of these individuals, the majority were Black/African American (45 percent) and Hispanic/Latino (31 percent).

People in their 20s and 30s are more likely to be diagnosed with HIV than those in other age groups. In 2021, 37 percent of new HIV infections occurred in people between the ages of 25 and 34, per HIV.gov.

The risk of HIV infection also varies by geographic region in the U.S. In 2021, more than half of new HIV infections were in the Southern states, per HIV.gov. Only 38 percent of the U.S. population live in the South.

What Should You Do Now?

It is important to use the different methods described here in order to prevent the spread of HIV. If you think you may have been exposed to HIV, you may need to seek emergency care and get tested right away. Be sure to discuss any positive test results or other questions you may have with your doctor. These are steps we can all take to help curb the spread of HIV and reduce the overall number of infections.

Find Your Team

On myHIVteam, the social network for people with HIV and their loved ones, more than 40,000 members come together to ask questions, give advice, and share their stories with others who understand life with HIV.

What questions do you have about causes and risk factors for HIV? What advice can you offer others? Share your experience in the comments below, or start a conversation by posting on your Activities page.