Cause and Risk Factors of HIV

This article is part of Health Divide: HIV, a destination in our Health Divide series.

HIV (human immunodeficiency virus) is a virus that can be passed from one person to the next through body fluids like semen, blood, vaginal secretions, and breast milk. HIV is most often transmitted during sex but can also be passed through shared needles. HIV can also be transmitted from the carrying parent to child during pregnancy, childbirth, or while breastfeeding.

Unlikely routes of transmission include blood transfusions, tattooing, and body piercing.

Other factors can increase the risk of transmission. Some of which are specific to certain groups because social factors, like discrimination and stigma, uniquely position them to be more likely to test HIV positive. These groups include Black people, Latinx people, trans people (especially trans women), men who have sex with men (MSM), and injecting drug users.

passionate couple is having sex on bed
Witthaya Prasongsin / Getty Images


HIV is a type of virus known as a retrovirus. It works by targeting immune cells called CD4 T cells that are responsible for instigating the immune response. When one of these cells is infected, the virus will "hijack" its genetic machinery and turn it into an HIV-producing factory. As the infected cell dies, the newly emergent viruses will go on to infect other CD4 T cells.

By progressively targeting and killing these cells, the body becomes less and less able to fight infections that it would otherwise be able to defend itself against.

When the immune defenses are compromised, serious and potentially life-threatening opportunistic infections can develop. This is the stage of infection commonly known as AIDS (acquired immune deficiency syndrome).

Even though HIV is transmitted through body fluids, not all body fluids are capable of transmitting the virus. Some fluids like saliva and urine contain high levels of enzymes or acids that prevent HIV from thriving. Once outside of the body, the virus is unable to survive for very long (and usually not at levels where transmission is likely).

Moreover, the virus must come in contact with mucosal tissues (such as found in the rectum or vagina) or enter the body through broken skin—and in sufficient quantities—to establish an infection.

HIV can be transmitted via:
  • Blood

  • Semen

  • Vaginal fluids

  • Rectal secretions

  • Breast milk

HIV is not transmitted via:
  • Saliva

  • Sweat

  • Tears

  • Shared utensils

  • Toilet seats

  • Pool or bath water

  • Mosquito bites

Sexual Risk Factors

The risk of HIV from sex varies by the type of sexual activity and other factors. The risk is greatest when condoms are not used. And, while it is possible not to get HIV after a single act of unprotected sex, people have gotten HIV after just one exposure.

According to the Centers for Disease Control and Prevention (CDC), the per-act risk of HIV from sex is as follows:

  • Receptive anal sex: 138 per 10,000 exposures (1.38%)
  • Insertive anal sex: 11 per 10,000 exposures (0.11%)
  • Receptive vaginal sex: 8 per 10,000 exposures (0.08%)
  • Insertive vaginal sex: 4 per 10,000 exposures (0.04%)
  • Oral sex: Risk is low to negligible

Additional Risk Factors

These per-act statistics may make it seem as if your general risk of HIV is extremely low, but they don't paint the complete picture. Other risk factors can amplify the risk, sometimes dramatically.

Among the risk factors to consider:

  • Anal sex: Anal sex is one of the most effective routes of transmission because rectal tissues are delicate and easily broken. Unlike the vagina, the rectal canal only has a single layer of cells overlying tissues that are rich in CD4 T cells.
  • Receptive sex: People with vaginas are more likely to get HIV from vaginal sex in part because the area of vulnerable tissues in the vagina is larger than those in the male urethra. The same applies to people who engage in receptive ("bottom") anal sex who are more likely to get HIV than their insertive ("top") partner.
  • HIV viral load: The HIV viral load is a test used to measure the amount of HIV in your blood, which can range from undetectable to well over a million. Not surprisingly, having a high viral load increases the risk of transmission, while a low viral load decreases the risk.
  • Sexually transmitted infections (STIs): STIs commonly occur with HIV and can facilitate infection in different ways. Certain STIs like syphilis cause ulcers that provide HIV easy access into the body. STIs also trigger an inflammatory response that draws immune cells to the site of the infection, providing more CD4 T cells for the virus to target and hijack.
  • Multiple partners: “Statistically, the more sexual partners you have, the greater opportunity there is to test positive for HIV. Moreover, a large sexual network can expose you to different drug-resistant HIV strains that may not respond as well to HIV medications.
  • Spermicides: Nonoxynol-9, a spermicide used to prevent pregnancy, can be irritating and trigger an inflammatory response that draws immune cells closer to the surface of the vagina. The Food and Drug Administration (FDA) currently advises against the use of nonoxynol-9 if a sexual partner has HIV.
  • Douching: Vaginal and anal douching strips tissues of mucus that provide some protection against HIV. There is debate as to how much this can increase a person's risk of infection, but the current evidence suggests that anal douching is potentially concerning.

Injecting Drug Use

The sharing of needles, syringes, and other drug paraphernalia is an effective way to pass HIV from one person to the next. The transmission of the virus is especially high when a person has a high viral load. Other practices such as "booting" or "kicking" (in which blood is drawn into the syringe and then injected) further amplify the risk.

According to the CDC, the risk of HIV from sharing needles is 63 per 10,000 exposures (0.63%). In certain countries, like Russia, injecting drug use is the predominant mode of HIV transmission, not sex.

Even beyond exposure to HIV-contaminated blood, people who are under the influence of drugs are more likely to engage in risky sexual behaviors, including unprotected sex, sex with multiple partners, or trading sex for money or drugs.

Since the 1990s, clean needle exchange programs have successfully reduced the rate of HIV and other communicable diseases such as hepatitis C by distributing clean syringes to drug users.

HIV and Injecting Drug Use in MSM and Trans Women

Men who have sex with men (MSM) are more than twice as likely to inject drugs than other men, most of whom are white and under age 35. Trans women and other transfeminine people are also more likely to use injection drugs compared to the general population. The drug methamphetamine is especially troubling as it increases the risk of HIV by 50% compared to other injected drugs (due in part to the hypersexual behaviors it can arouse).

Pregnancy and Breastfeeding

Pregnancy is a less common mode of infection but one that still poses risks to certain groups. There are three circumstances under which a birthing parent with HIV can pass the virus to their baby: during pregnancy, delivery, breastfeeding, or chestfeeding (feeding a baby milk from your chest).

The transmission of the virus before or during childbirth is largely the result of the rupture of the placenta which exposes the baby to HIV-positive blood. HIV is also present in chest milk or breast milk and can potentially cause infection, particularly if the birthing parent does not have an undetectable viral load.

In the United States, birthing parent-to-child transmission of HIV is rare, thanks to routine prenatal screening and the use of antiretroviral drugs which can reduce the risk of transmission to less than 1%. Even if a birthing parent arrives at the hospital with no prior prenatal care, an intravenous infusion of antiretroviral drugs used in combination with a cesarean section can greatly reduce the risk of transmission.

In the United States, parents are advised not to breastfeed if they have HIV and to bottle-feed instead. In developing countries, the benefits of exclusive breastfeeding (in terms of nutrition and the building of immunity) are seen to outweigh the risks in resource-poor communities.

In 2014, only 33 reported cases of birthing parent-to-child transmission of HIV were reported in the United States.

Pregnancy Risks Among Black Women With HIV

Despite the low number of mother-to-child transmissions in the United States, the risk among Black women is eight times greater than in White women and seven times greater than in Latinx women. Moreover, 60% of children living with HIV are born to Black mothers with HIV.

Needlestick Injuries

Needlestick injuries (sometimes referred to as sharps injuries) can expose a person to tainted blood or body fluids. The risk is of concern for doctors, nurses, and other healthcare workers who are vulnerable to blood-borne infections from patients.

Even so, the CDC suggests that the risk is low due to universal precautions that are designed to reduce the risk of disease transmission to healthcare workers. Today, the per-incidence risk of HIV from a needlestick injury is roughly 0.23%.

If an accident does occur, a preventive strategy called post-exposure prophylaxis (PEP) may avert an infection if started within 72 hours of the exposure. PEP involves a 28-day course of antiretroviral drugs and may reduce the likelihood of HIV from a high-risk exposure by 81%.

Blood Transfusions

In the early days of the AIDS pandemic, a blood transfusion was a common cause of HIV infection. With the institution of HIV blood screening in 1985 and universal blood screening in 1992, the likelihood of transmission by this route is almost unheard of.

In fact, according to the CDC, there were only three confirmed cases of HIV out of an estimated 2.5 million blood transfusions from 1999 to 2013.

Similarly, state regulations governing the licensing of tattoo and piercing parlors have also made these procedures unlikely causes of HIV. To date, there are no documented cases of HIV transmission from either tattooing or body piercing. The CDC considers the risk negligible.

A Word From Verywell

Understanding the causes and risk factors of HIV can help you develop strategies to protect yourself whether you have HIV or don't. This first step is getting diagnosed and knowing your HIV status.

If you don't have HIV, you can avoid infection by using condoms, reducing your number of sex partners, or using a preventive strategy called pre-exposure prophylaxis (PrEP), which can reduce your risk of HIV by as much as 99%.

If you have HIV, early treatment can cut your risk of serious HIV- and non-HIV-related illness by more than half while allowing you to enjoy a normal to near-normal life expectancy. Moreover, having an undetectable viral load reduces your risk of infecting others to zero.

Frequently Asked Questions

  • What is the cause of HIV?

    HIV stands for the human immunodeficiency virus. The virus causes disease by targeting and killing immune cells, called CD4 T-cells, that are responsible for coordinating the immune defense. As these cells are progressively destroyed, the body becomes less and less able to defend itself against infections that it could otherwise control.

  • What causes AIDS?

    "AIDS" is the term used to describe the stage in which the immune system is compromised by HIV infection. Traditionally, AIDS is diagnosed when the CD4 count falls below 200 or when you get any one of the 29 different diseases classified as AIDS-defining conditions.

  • How do you get HIV?

    Most people get HIV through sex, namely vaginal or anal sex. HIV can also be passed through shared needles or needlestick injuries in healthcare settings. Mothers with HIV can pass the virus to their babies during pregnancy (although this is less common in the United States due to the routine testing and treatment of mothers during perinatal care).

  • What are the ways you cannot get HIV?

    You cannot get HIV from kissing, touching, hugging, toilet seats, shared utensils, or mosquito bites. In the past, there was a risk of getting HIV from a blood transfusion, but this rarely occurs in the United States due to routine blood screening. Tattooing and piercing can theoretically transmit HIV, but there has yet to be a documented case of this occurring in the United States.

  • What is at risk for HIV?

    The risk of getting HIV is higher for:

    • People who engage in sex without condoms
    • People with multiple sex partners
    • Injecting drug users who share needles
    • Black and Latinx people
    • Trans people, especially trans women
    • Men who have sex with men (MSM)
    • Having sex with someone who shares needles or has multiple sex partners
    • Having another sexually transmitted infection (STI)
  • Can you get HIV from oral sex?

    Theoretically, you can, but there are few documented cases of this occurring. This is partly because the environment in the mouth is hostile to the virus and the tissues of the mouth aren't as susceptible as some other areas of the body. Even so, other STIs can be passed through oral sex, but they may be avoided with condoms or dental dams.

  • Why are Black trans people, especially Black trans women, and gay men more likely to test HIV positive?

    There are multiple factors that contribute to this. Transphobia, homophobia, and HIV stigma contribute to high rates of denial, secrecy, and depression. This, in turn, increases the risk of substance or alcohol abuse and risk-taking behaviors. Black people also experience higher rates of criminalization and homelessness in comparison to the general population. These factors are additive and result in a higher likelihood of testing positive for HIV in one’s lifetime. The extreme vulnerability of rectal tissues and smaller gay and trans sexual networks due to the reasons stated above also contribute.

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Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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By Mark Cichocki, RN
Mark Cichocki, RN, is an HIV/AIDS nurse educator at the University of Michigan Health System for more than 20 years.