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).

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).