Some modes of transmission are more efficient than others. In order for HIV to be transmitted, the virus needs to come into contact with porous mucous membranes (such as those found in the rectum and vagina), pass through breaks and tears in tissues (such as can occur during intercourse), or enter the bloodstream directly (such as through shared needles).

Moreover, there is needs to be ample quantities of the virus to breach the body’s frontline immune defenses. This is why HIV cannot be passed through saliva, the environment of which is hostile to the virus, or when the virus is fully suppressed (undetectable) in an HIV-positive person on antiretroviral therapy.

There are four conditions that must be met for an HIV infection to take place:

There must be body fluids in which the virus can thrive. HIV cannot thrive in the open air or in parts of the body with a high acid content, such as the stomach or bladder. There must be a route of transmission by which the virus enters the body. Primary routes of transmission include sexual intercourse, shared needles, and mother-to-child infections. There must be immune cells present near the site of entry. This allows the virus to take hold once it has entered the body. There must be sufficient amounts of the virus in the body fluids. These amounts, measured by the viral load, can be high in body fluids like blood and semen and low to non-existence in tears and saliva.

Anal Sex

Anal sex is the predominant means of HIV transmission in the United States, occurring at a rate 18 times greater than that of vaginal sex. There are a number of reasons as to why this is, not least of which is the fact that rectal tissues are more fragile and vulnerable to rupture than vaginal tissues.

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Little microtears that commonly occur during anal intercourse simply allow more viruses to enter the body. They also expose the receptive partner’s potentially infected blood to the insertive partner, thereby transmitting to the insertive partner. On top of that, many people who engage in anal sex will often douche before intercourse, stripping away the layer of mucus that might impede HIV transmission.

These vulnerabilities help explain why infection rates in the United States are highest among men who have sex with men (MSM). But the risk is not only limited to gay and bisexual men; anywhere from 16% to 33% of heterosexual couples engage in anal sex as well, often without condoms.

The risk is further increased if the insertive partner is uncircumcised, as microbes beneath the foreskin can increase the shedding (expulsion) of the virus into seminal fluids.

Vaginal Sex

Vaginal sex is the second most common mode of HIV transmission in the United States. In many parts of the developing world, vaginal sex is the primary mode of transmission, with women disproportionately affected compared to men.

Women are more vulnerable for several reasons:

The area of exposure within the vagina is greater than that of the penis. The vagina and cervix are vulnerable to common infections like bacterial vaginosis and candidiasis (yeast infection), both of which compromise already delicate tissues. During unprotected sex, the ejaculate can often remain inside the vagina for a prolonged period of time. Vaginal douching prior to sex can strip away protective mucus.

This is not to suggest that the male partner is without risk, especially uncircumcised men. The bacteria-rich environment beneath the foreskin helps facilitate infection by increasing the number of white blood cells, called Langerhans cells, that reside within the skin itself. These cells can inadvertently “capture” HIV and pull them into the body.

Sexually transmitted diseases (STDs) like chlamydia, gonorrhea, and syphilis can further enhance the risk in men and women, either by increasing viral shedding in people with HIV or compromising genital tissues in those without.

Oral Sex

Oral sex is an inefficient way of transmitting HIV, whether it be oral-penile sex (“blowjobs”), oral-vaginal sex (cunnilingus), or oral-anal sex (“rimming”). The current scientific consensus is that HIV transmission among those who engage exclusively in oral sex is unlikely. The risk may not be zero, but most agree that it is edging close to that.

A 2014 study in the journal AIDS suggests that the per-act risk of infection from oral sex between an HIV-negative receptive partner and an HIV-positive insertive partner hovers between 0% and 0.4%.

This is not to suggest that people can have oral sex with impunity. Cuts, abrasions, and sores on the genitals or in the mouth can potentially increase the risk of infection, as can STDs or menstruation (both of which promote HIV shedding).

Other STDs besides HIV can also be transmitted through oral sex, including chlamydia, gonorrhea, herpes, human papillomavirus (HPV), and syphilis. Getting an STD independently increases the risk of HIV.

Injecting Drug Use

Sharing injecting needles is an extremely effective way of transmitting HIV, directly inoculating the virus from the blood of one person into that of another.

In the United States, the growing opioid crisis has spurred an increase in HIV infections. One of the most publicized HIV outbreaks occurred in 2015 when 79 infections were reported in the town of Austin, Indiana (population 4,295), all of which were attributed to the shared use of needles among recreational oxymorphone users.

Even among anabolic steroid users, there has been an increase in the number of people who share needles, with nearly one in 10 getting HIV according to a 2013 study in the British Journal of Medicine.

Blood Transfusions and Transplants

In the early days of the HIV epidemic in the 1980s to early 1990s, there were many people infected with HIV due to tainted blood transfusions. Prior to 1992, there were no screening tools available to ensure that the U.S. blood supply, including clotting factors and plasma, was free of the virus.

That risk has fallen dramatically in recent decades due to advances in detection technologies and the universal screening of blood and tissue donations in the United States and other countries. This not only includes the screening of HIV but other bloodborne infections like hepatitis B and hepatitis C.

The risk outside of the United States can vary dramatically. In Egypt, for instance, one in four HIV infections is the result of a transfusion. By contrast, in South Africa, the country with the highest HIV incidence in the world, the transmission risk is closer to one of every 76,000 transfusions.

Pregnancy

As with blood transfusions, the risk of mother-to-child HIV infection was high in the early years of the global pandemic. Today, the risk has dropped dramatically, even in hard-hit parts of Africa, due to routine HIV screening in pregnant people and the use of antiretroviral drugs to prevent vertical (mother-to-child) transmission.

When HIV transmission does occur, it usually happens during childbirth with rupture of membranes, which exposes the baby to HIV-tainted blood and vaginal fluids. Prior to this, HIV generally does not cross the placenta from mother to child unless there is placental abruption, the premature rupture of membranes, or a similar problem.

Even so, the use of antiretrovirals in the pregnant person can reduce the risk of vertical transmission by as much as 95% by suppressing the virus to undetectable levels.

HIV can also be transmitted through breast milk, and, in the United States, people with HIV are routinely advised to avoid breastfeeding irrespective of whether they are on HIV therapy or have an undetectable viral load. (The same recommendations are not extended to the developing world, where the benefits of infant nutrition are seen to outweigh the risks.)

Annually, there are only about 150 cases of vertical transmissions in the United States, most often caused when a person presents at the hospital late in pregnancy or fails to adhere to HIV therapy.

Other Possible Causes

There are other, less common causes of HIV transmission and several for which the risk of HIV is unlikely but possible. These include occupational exposure, dental procedures, body piercings and tattoos, and shared sex toys.

Occupational Exposure

HIV transmission from needlestick injuries or other occupational exposures can place healthcare workers at risk. With that said, the risk of HIV from a needlestick injury is less than one in 1,000, while contact with HIV-infected body fluids on intact skin is even lower.

To date, only 58 cases of occupational HIV transmissions have been confirmed. Others are believed to have been averted with a 28-day course of antiretrovirals called HIV post-exposure prophylaxis (PEP).

Dental Procedures

Back in the early days of the AIDS crisis, headlines were made when a Pennsylvania woman named Kimberly Bergalis claimed to have gotten HIV from a dental procedure. The claims were considered dubious given that Bergalis failed to report prior sexually transmitted infections.

Since that time, there have been several claims but no documented cases of HIV transmission among dental patients. Of eight cases reported among dentists, none have been confirmed as having been transferred during a dental procedure.

Body Piercings and Tattoos

While theoretically feasible, the risk of HIV from body piercings and tattoos is low due to the licensing and strict regulation of practitioners within the industry. For its part, the CDC insists that the risk of HIV transmission is low to negligible.

Among unlicensed practitioners who do not adhere to industry sterilization and hygiene practices, the risk is potentially higher, although it is unclear by how much.

Shared Sex Toys

Sex toys are generally regarded as a form of safer sex. With that said, shared insertive toys like dildos are regarded as potentially unsafe due to exposure to blood and other bodily fluids.

To date, the risk of HIV from shared toys remains unclear since toys are rarely the only form of sex a couple will engage in. The same applies to fisting and other sexual practices that disrupt or traumatize rectal or vaginal tissues. These activities can theoretically potentiate infection, but studies have yet to confirm this.

Factors that Increase Risk of Transmission

There are a number of factors that can significantly increase the risk of HIV transmission irrespective of the route of exposure:

Unprotected sex: Simply put, using a condom reduces the risk of HIV transmission by roughly 95%. Not using a condom erases that protective benefit. High viral load: Every ten-fold increase in viral load—from, say, 1,000 to 10,000 to 100,000—increases your risk of HIV by two- to three-fold. Taking antiretroviral therapy reduces that risk. Multiple partners: Having multiple sex partners increases your opportunity for HIV exposure. Even if you think a partner is “safe,” serosorting (choosing a partner based on their presumed HIV status) is associated with a three-fold risk of getting an STD. Substance abuse: Beyond the risk of HIV from shared needles, illicit drugs like crystal methamphetamine and heroin can impair judgment and increase risk-taking. Even non-injecting drugs and alcohol can lead to sexual disinhibition and risk-taking. Sexually transmitted diseases: STDs increase the risk of getting and transmitting HIV. With ulcerative STDs like syphilis, the risk of HIV may increase by as much as a 140-fold in high-risk MSM populations. Genital infections: Non-sexually-acquired genital infections carry similar risks of transmission. Even uncomplicated ones like urethritis are associated with an eightfold increase in the risk of HIV. Douching: Some studies have shown that rectal douching in high-risk MSM populations more than double the risk of HIV from 18% to 44%. The risk of HIV from vaginal douching is less clear but is known to increase the risk of bacterial vaginitis. Being a sex worker: The more people one has sexual encounters with, the more likely the possibility of sexual transmission is. Where you live: Living in dense urban populations where HIV prevalence rates are high places you at greater risk compared to rural settings. This is especially true in poorer, ethnic neighborhoods where access to treatment and preventive services are lacking.

A Word From Verywell

Understanding the risks of HIV can help you build individual strategies to help prevent either getting or spreading HIV. This may involve the consistent use of condoms and a reduction in the number of sex partners or the use of needle exchange programs and other harm reduction strategies if you inject drugs.

If you have HIV, arguably the best way to prevent transmission is by sustaining an undetectable viral load with antiretroviral therapy. Studies have proven that doing so reduces the risk of HIV transmission to zero.

If you don’t have HIV, you can protect yourself by taking HIV pre-exposure prophylaxis (PrEP), a type of antiretroviral medication that can reduce your risk of infection by up to 90% if taken as prescribed.