Cause and Risk Factors of HIV

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HIV is an infection caused by the human immunodeficiency virus (HIV), which is transmitted through blood, semen, and certain other bodily fluids. For this reason, the primary risk factors for infection are related to lifestyle behaviors such as having unprotected sex and sharing needles to inject recreational drugs. The virus also can be passed from a mother to her baby during pregnancy, childbirth, or through breastmilk. Having another sexually transmitted infection (STI) is also associated with the risk of HIV transmission.

That scientists ultimately were able to pinpoint exactly how HIV is transmitted is the silver lining of the health crisis that began in the early 1980s: This understanding has been a vital part of educating the general public about the many ways to prevent the spread of HIV.

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The human immunodeficiency virus that causes HIV infection is classified as a retrovirus. It causes disease by infecting and destroying blood cells known as CD4 T-cells that play a key role in the immune system. As these cells are progressively wiped out, the body becomes less and less able to defend itself.

If untreated, HIV will progressively wipe out these cells, making the body less and less able to defend itself against opportunistic infections—so called because they take advantage of the body's weakened immune response—and ultimately leading to a fatal stage of infection commonly known as acquired immunodeficiency disease (AIDS).

Fortunately, it's rare for HIV to reach this stage in the United States and other developed countries because of access to medications for managing it.

It's important to understand that, although the human immunodeficiency virus can be transmitted in some fluids that people frequently exchange, there are other fluids in which the virus does not thrive. This has been a source of confusion and unnecessary fear in the past.

HIV can be transmitted via:
  • Blood

  • Pre-seminal fluid

  • Semen

  • Vaginal fluids

  • Rectal fluids

  • Breast millk

HIV is not transmitted via:
  • Saliva

  • Sweat

  • Tears

  • Shared foods or beverages

  • Toilet water

  • Pool or bath/shower water

It's also helpful to understand that for any these fluids to cause an infection, they must come in contact with a mucous membrane (found inside the rectum, vagina, penis, and mouth), damaged tissue, such as an open wound, or be directly injected into the bloodstream.

Risk Factors

The behaviors most commonly associated with HIV infection are unprotected sexual contact and injectable drug use.

Sexual Contact

The risk of getting HIV from having sex with a person infected with HIV depends on the type of sexual activity. According to the U.S. Centers for Disease Control and Prevention (CDC), these include:

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

There have been some reports of women who have sex with women passing the virus along in menstrual blood or vaginal fluids, but this is very rare.

The higher the levels of HIV in an infected person's bloodstream (the HIV viral load), the more likely they are to pass it along to a sexual partner. Note that having an STI can greatly increase a person's chance of acquiring HIV. What's more, someone who is HIV-positive and has a co-existing STI is more likely to pass the human immunodeficiency virus to someone else.

Conversely, there are a number of variables that can lower a person's chance of acquiring or passing along HIV during a sexual encounter.

The use of both antiretroviral therapy and condoms decreases a person's risk of getting HIV after a sexual exposure by more than 99%. In addition, pre-exposure prophylaxis (PrEP) may lower the risk of HIV infection by more than 90% for certain groups of people.

Injectable Drug Use

The HIV virus can be spread by being directly introduced into the bloodstream. It follows, then, that sharing contaminated needles or syringes, other drug paraphernalia, or rinse water with a person who's HIV-positive is dangerous behavior. According to the CDC, the risk of transmitting HIV by sharing needles is 63 per 10,000 exposures (0.63%).

Since the early- to mid-1990s, syringe services programs (SSPs), also known as needle exchange programs (NEPs), have successfully reduced the rate of HIV and other communicable diseases such as hepatitis C by distributing clean syringes to drug users.

Blood Transfusions

The most direct way to become infected with HIV is to receive a transfusion of infected blood. Under these circumstances, that risk of infection is around 93%.

However, thanks to advanced blood screening techniques that make it possible to detect HIV in blood donors, this scenario is rare.

According to the CDC, from 1999 to 2013, only three of an estimated 2.5 million blood recipients were confirmed to have acquired HIV from a blood transfusion due to a false-negative reading.

Needlestick Injuries

Needlestick injuries (sometimes referred to as sharps injuries)—as well as any under-the-skin injury that can expose a person to tainted blood or body fluids—have long been a concern, particularly for doctors, nurses, and other healthcare workers.

Training and taking precautions can go a long way toward lowering the risk of HIV infection from a needlestick, a risk the CDC puts at 0.23%. However, accidents do happen. A practice called post-exposure prophylaxis (PEP), in which medication is given within 72 hours of an unintended needlestick, has been found to lower the risk of viral transmission by approximately 81%.

Contaminated Body Piercing or Tattoo Equipment

Although rare, it's possible to be exposed to HIV in a piercing or tattoo establishment that reuses or does not sanitize needles, ink, and other supplies.

Before having any sort of body art procedure that involves needles or skin punctures, make sure the practitioner is licensed and follows strict sanitation guidelines.

Mother-to-Child Transmission

There are three circumstances under which a woman who is HIV-positive can pass the virus along to her baby: during pregnancy, delivery, or breastfeeding.

In the United States, mother-to-child transmission of the virus is rare, thanks to antiretroviral drugs which, when taken during pregnancy, can decrease the risk of transmission to less than 1% if the viral load is adequately suppressed (to undetectable levels). Bottle-feeding, rather than nursing, will further protect a baby born to a mother who is HIV-positive.

Likewise, the expanded distribution of HIV drugs in the developing world has led to enormous reversals in some of the hardest hit countries in Africa. The women in these countries may be an exception to the no-nursing rule, however, as the many health benefits of human milk outweigh the risk of infection in these populations.

A Word From Verywell

Understanding the behaviors and circumstances that increase the risk of contracting or transmitting HIV is the first step in protecting your health and that of others. When they cannot be avoided, they should be engaged in safely. This means asking if potential sexual partners are HIV-positive, being tested if you're at risk of being positive, using condoms during sex, and not sharing needles if you use injectable drugs.

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