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.
Causes
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.
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.
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.
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.
People who engage in sex without condomsPeople with multiple sex partnersInjecting drug users who share needlesBlack and Latinx peopleTrans people, especially trans womenMen who have sex with men (MSM)Having sex with someone who shares needles or has multiple sex partnersHaving another sexually transmitted infection (STI)
