HIV TRANSMISSION AND RISK FACTORS

An individual can only become infected with HIV through direct contact with certain body fluids from a person with HIV who has a detectable viral load. A detectable viral load is having more than 200 copies of HIV in a milliliter of blood.

Transmission Routes

Body fluids known to transmit HIV include:

Blood
Semen and preseminal fluids
Rectal fluids
Vaginal fluids
Breast milk

In addition, any body fluid visibly contaminated with blood should be considered capable of transmitting HIV. These fluids may include:

Cerebrospinal
Amniotic
Pleural
Synovial
Peritoneal
Pericardial

For transmission to occur, the virus must enter the bloodstream of an HIV-negative person through a mucous membrane. These are located in the:

Rectum
Vagina
Mouth
Tip of the penis

The virus can also be transmitted through open cuts or sores or through direct injection (e.g., by a needle or syringe).

Unless blood is visibly present, HIV cannot be transmitted by:

Saliva
Sputum
Sweat
Tears
Feces
Nasal secretions
Urine
Vomitus

HIV also cannot be transmitted by:

Air
Water
Donating blood
Closed-mouth kissing
Insects
Pets
Sharing food or drinks
Sharing toilets

The main routes of HIV transmission are through:

Unprotected sexual contact with an infected person
Sharing needles and syringes with an infected person
From an infected mother to child during pregnancy, during birth, or after birth while breastfeeding

Additional criteria for HIV transmission to occur include:

HIV must be present in sufficient transmittable amounts.
HIV must be able to enter the bloodstream of the next person.

HIV is a fragile entity and cannot survive for a substantial amount of time in the open air. The length of time HIV can survive outside the body is dependent on the amount of virus present in the body fluid and the conditions the fluid is subjected to. Studies have shown that when a high level of HIV that has been grown in a lab is placed on a surface, it loses most of its ability to infect (90%–99%) within several hours, indicating that contact with dried blood, semen, or other fluids poses little risk (HIV.gov, 2022a; St. Maarten AIDS Foundation, 2023).

SEXUAL CONTACT

HIV transmission rates vary by the type of sexual contact. The chances of contracting HIV after one exposure are highest among those who have receptive anal sex (about 1%). This means that someone can get the virus 1 out of every 100 times they have receptive anal sex without a condom. The reason for the higher risk of transmission by anal sex is due to the thin layer and easy penetrability of the cells in the anus compared to the vagina. HIV probability is lower for those having insertive anal sex, followed by receptive and insertive vaginal sex. With all three types of sex, the odds of contracting HIV after one exposure are well below 1% (Watson, 2022).

HIV acquisition rates among uncircumcised males are higher than for circumcised males. This may be related to a high density of HIV target cells in the male foreskin, including Langerhans cells and macrophages. Circumcision reduces the risk of female-to-male HIV transmission by 50%–60%; however, circumcision does not appear to decrease the risk of HIV transmission to the partner (Cohen, 2022).

INJECTION DRUG USE

Sharing injection needles, syringes, and other paraphernalia with an HIV-infected person can send HIV (as well as hepatitis B and C viruses and other bloodborne diseases) directly into the user’s bloodstream. About 1 in 10 new HIV diagnoses in the United States is attributed to injection drug use or the combination of male-to-male sexual contact and injection drug use. The risk is high because needles, syringes, or other injection equipment may be contaminated with blood, which can survive in a used syringe for up to 42 days, depending on temperature and other factors. HIV-negative persons have a 1 in 160 chance of getting HIV every time they use a needle that has been used by someone else who has HIV. Sharing syringes is the second riskiest behavior following receptive anal sex (CDC, 2021b).

BLOOD TRANSFUSION

The chances that donated blood will contain HIV is less than 1 in nearly 2 million. While all blood donations are screened for HIV before they enter the blood pool, all laboratory tests have a “window period” in which very recent HIV infections cannot be detected, and in those most sensitive assays that are used by blood collection agencies, this window may be between 10–16 days. Because of this, a small number of infected samples still make it through testing (Tufts Medical Center, 2022).

TATTOOING, BODY PIERCING

There are no known cases in the United States of anyone becoming infected with HIV from professional tattooing or body piercing. There is, theoretically, a potential risk, especially during the time period when healing is taking place. It is also possible to become infected by HIV from a reused or not properly sterilized tattoo or piercing needle or other equipment, or from contaminated ink. The risk is very low but increases when the person doing the procedure is not properly trained and licensed (CDC, 2022b).

MOTHER-TO-CHILD TRANSMISSION

Before effective treatment was available, about 25% of pregnant mothers with HIV passed the virus to their babies. Today, if the mother is receiving HIV treatment and has a sustained undetectable viral load through pregnancy and postpartum, the risk of passing HIV to the baby is less than 1%. The risk of HIV transmission while breastfeeding is also less than 1% for women with HIV on antiretroviral therapy with sustained undetectable viral load through pregnancy and postpartum (USDHHS, 2021; NIH, 2023).

At Risk Populations and Behaviors

HIV does not discriminate. Anyone can become infected with the virus. However, there are certain groups of people who are more likely to get HIV than others. This is due to factors such as the communities in which they live, what subpopulation they belong to, and any risky behaviors they become involved in.

COMMUNITIES

Communities at high risk for the spread of HIV are very diverse and can include college campuses, the military, gay neighborhoods, crack houses, prisons, bathhouses, brothels, neighborhoods with homeless people, and “shooting galleries” (locations where intravenous drug users can rent or borrow needles and syringes). The level of risk within these communities varies; however, HIV usually spreads rapidly due to the existence of tightly linked networks connected through sexual behavior and the use of drugs (NAS, 2023).

MEN WHO HAVE SEX WITH MEN

Men who have sex with men (MSM) are disproportionately at risk for HIV infection. In the United States, the estimated lifetime risk for HIV infection among MSM is 1 in 6, compared with 1 in 524 among heterosexual men and 1 in 253 among heterosexual women. These inequalities are further intensified by race and ethnicity, with African American/Black and Hispanic/Latinx MSM having a 1 in 2 and a 1 in 4 lifetime risk, respectively (CDC, 2021c).

RACIAL AND ETHNIC MINORITIES

In comparison to their percentage of the population, some groups have higher rates of HIV infection in their communities, which raises the risk of new infections with each sexual or injection drug use encounter.

In 2019, Black/African American people represented 13% of the U.S. population but 40% of those with HIV. Hispanic/Latinx people represented 18.5% of the population but 25% of people with HIV. Black women are overly affected by HIV as compared to women of other races/ethnicities, with the rate of new infections 11 times that of White women and four times that of Latina women. This unequal impact on these communities is also apparent in the incidence of new HIV infections and shows that effective prevention and treatment are not adequately reaching people who could benefit the most.

Additionally, among these groups, a range of social, economic, and demographic factors—such as stigma, discrimination, income, education, and geographic region—affect risk for HIV. These factors help to explain why African Americans have worse outcomes on the HIV continuum of care, including lower rates of linkage to care and viral suppression (HIV.gov, 2023a; CDC, 2023a).

PERSONS WHO INJECT DRUGS

People who inject drugs account for about 1 in 10 HIV diagnoses in the United States. An individual who is HIV-negative has a 1 in 160 chance of getting HIV each time they use a needle that has been used by someone with HIV. Sharing syringes is the second-riskiest behavior for infection with HIV.

Using drugs itself can also increase risk for HIV infection. People who are under the influence of a substance are more apt to engage in risky sexual behaviors, such as having unprotected anal or vaginal sex, having multiple sex partners, or trading sex for money or drugs (CDC, 2022c).

PERSONS WHO EXCHANGE SEX FOR MONEY OR NONMONETARY ITEMS

This group includes a broad range of individuals who trade sex for income or other items such as food, drugs, medicine, and shelter. They are at higher risk for HIV because they are more likely to engage in risky sexual behaviors and substance use. Those who exchange sex more regularly as a source of ongoing income are at higher risk than those who do so infrequently. This includes those working as escorts, in massage parlors, in brothels, in the adult film industry, as state-regulated prostitutes (in Nevada), and anyone trading sex to meet their basic needs. For any of these people, sex can be consensual or nonconsensual (CDC, 2022d).

INCARCERATED PERSONS

More than 2 million people in the United States are incarcerated in federal, state, and local correctional facilities on any given day. Prisoners are five times more likely to be infected with HIV than other populations. Only 7% of incarcerated people are women, and HIV prevalence among women in prison is 4% compared to 3% in men.

One reason incarcerated people are at higher risk of HIV involves the difficulty in obtaining clean injecting or tattooing equipment in prisons, since having a needle is often a punishable offence. Therefore, people share equipment to take drugs or tattoo other prisoners, and this is one of the primary causes of HIV infection in prisons.

Sex is also often forbidden in prisons, but it does happen. Prevalence of sexual activity is largely unknown and considered to be significantly underreported due to denial, fear of stigma, and homophobia. In addition, condoms are often not available to prisoners. This means sexual activity is most often unprotected (Be in the Know, 2023a).

OLDER ADULTS

In the United States, HIV infection in both women and men ages 50 and older is most commonly acquired through heterosexual transmission. Certain age-related issues may cause older females to be at higher risk for HIV acquisition, such as vulvovaginal atrophy. Aging females are also less likely to use a condom to prevent pregnancy, which puts them at risk for HIV acquisition if they enter a new sexual partnership.

Injection drug use is also an important but less common risk factor. Twenty-four percent of HIV-negative people ages 50 or older who inject drugs used a syringe that someone else had used (Greene, 2023).

WOMEN AND GIRLS

Nearly 1 in 5 U.S. women has been raped in her lifetime, and 2 in 5 women have experienced another type of sexual assault. Violence against females plays a role in transmitting HIV infection. Types of violence include forced sex and sexual abuse in childhood. Forced sex can cause tears or cuts, allowing easy entry of HIV. This is especially true for girls and younger women whose reproductive tracts are not fully developed. Additionally, the male may not use a condom while engaging in sexual violence. Sexual abuse in childhood raises the lifetime risk of HIV infection, and women who were sexually abused are more likely to report risk-taking behaviors later in life (OWH, 2021).

ADOLESCENTS

Most adolescents ages 13 and older infected with HIV acquired it through sexual activity. Young male adolescents who have sex with men account for 82% of new HIV infections in this age group. Although most adolescents acquire HIV infection through sexual activity, there remains a small proportion who were infected prenatally.

Black adolescents accounted for an estimated 52% of all new HIV infections among this population, followed by Hispanic/Latinx (25%) and White (18%). Geographically, southern states have the greatest percentage (>50%) of adolescents affected by HIV.

The CDC reports that 48% of adolescents reported not using a condom the last time they had sex, and 8% had been physically forced to have sexual intercourse when they did not want to. Data show a decline in sexual risk behaviors among high school students, with fewer students currently being sexually active and fewer having ever had sex (30%).

Among LGBTQ+ adolescents, 1 in 5 experienced sexual violence compared to 1 in 10 of their heterosexual peers. LGBTQ+ adolescents (21%) were also more likely to have ever used illegal drugs than heterosexual students (12.7%) (CDC, 2023b, 2019; Gillespie, 2023).

TRANSGENDER PERSONS

In 2022, over 1.6 million adults (ages 18 and older) and youth (ages 14–17) identified as transgender in the United States. In 2019, transgender people made up 2% of new HIV diagnoses. Studies have found that transgender women have 39 times the odds of being infected with HIV compared to the general population. Transgender men also have higher rates of infection.

Risk factors that may contribute to high infection rates include higher rates of sexual violence, drug and alcohol abuse, sex work, incarceration, homelessness, attempted suicide, unemployment, lack of familial support, violence and sexual violence, stigma and discrimination, limited healthcare access, and negative healthcare encounters (HRCF, 2022; Herman et al., 2022).

Other Factors Affecting Transmission Risk

Many other factors, alone or in combination, affect the risk of HIV transmission.

HIGH VIRAL LOAD

The higher someone’s HIV viral load, the more likely the person is to transmit HIV. Viral load is highest during the acute phase of HIV and without HIV treatment.

A high HIV viral load is generally considered to be above 100,000 copies per milliliter of blood, but a person could have 1 million or more. At this point the virus is at work making copies of itself and the disease may progress quickly.
A lower viral load is below 10,000 copies per milliliter of blood. At this point, the virus isn’t actively reproducing as fast and damage to the immune system may be slowed.
An undetectable viral load is generally considered to be less than 20 copies per milliliter of blood. At this point the virus is suppressed. This does not mean, however, that there is no virus in the body; it just means there is not enough for the test to detect and count. People with HIV who achieve and maintain an undetectable viral load cannot transmit HIV sexually to their partners.
(IAPAC, 2021)

OTHER SEXUALLY TRANSMITTED DISEASES/INFECTIONS (STDs/STIs)

People who have a sexually transmitted disease (also called sexually transmitted infection [STI]) may be at an increased risk of acquiring or transmitting HIV. Some of the most common STDs include gonorrhea, chlamydia, syphilis, trichomoniasis, human papillomavirus (HPV), genital herpes, and hepatitis.

One reason for this is that the behaviors that put people at risk for one infection often put them at risk for others. When a person with HIV acquires another STD such as gonorrhea or syphilis, it is likely they were having sex without using condoms. Also, STDs and HIV tend to be linked, and when someone gets an STD, it indicates they may have acquired it from someone who may be at risk for other STDs as well as HIV.

People with HIV are more likely to shed the virus when they have urethritis or a genital ulcer, and in a sexual partner, a sore or inflammation caused by an STD may allow infection that would have normally been stopped by intact skin. Even STDs that cause no breaks or open sores can increase the risk by causing inflammation that increases the number of cells that can serve as targets for HIV.

Both syphilis and HIV are highly concentrated among gay, bisexual, and other men who have sex with men (MSM). In 2021, MSM only and men who have sex with both men and women accounted for 63% of all primary and secondary syphilis cases in which the sex of the partner was known. HIV is more closely linked to gonorrhea than chlamydia (common among young women), and herpes simplex (HSV-2) is commonly associated with HIV. Studies have shown that persons infected with herpes are at three-times higher risk for acquiring HIV infection (CDC, 2023c).