How is HIV Transmitted?
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Human immunodeficiency virus (HIV) was established as the cause of the acquired immunodeficiency syndrome (AIDS) in 1983. Ever since then, a lot of research has been conducted and a great deal of information has been generated regarding the ways HIV can be transmitted from one person to another.

The problem with much of the information about HIV transmission, especially on the Internet, is that it speaks in very general terms. All too often, advice from one site will directly contradict advice from another site as well. For example, some sources refer to oral sex as "risky," whereas others say it is "low risk" or "no risk." This can be very frustrating and it also leads to the spread of misinformation, and frequently a lot of unnecessary worry, about the transmission of HIV.

HIV infection—and HIV testing—is a medical issue. We have developed this lesson to provide straightforward and accurate information regarding HIV transmission. However, it is important to stress that this lesson—and other sources of HIV information on the Internet—should not be consulted as an alternative to medical care and testing. If you fear that you have been exposed to HIV—regardless of how low the perceived risk and no matter how much information you find on the Internet—you need to get in touch with your health care provider or an HIV testing center.

How is HIV Transmitted?

HIV enters the body through open cuts, sores, or breaks in the skin; through mucous membranes, such as those inside the anus or vagina; or through direct injection. There are several ways by which this can happen:

Sexual contact with an infected person. This is the primary focus of this lesson and is reviewed in greater detail in the following sections.
Sharing needles, syringes, or other injection equipment with someone who is infected.
Mother-to-child transmission. Babies born to HIV-positive women can be infected with the virus before or during birth, or through breastfeeding after birth. More information about HIV and pregnancy can be found in this lesson.
Transmission in health care settings. Healthcare professionals have been infected with HIV in the workplace, usually after being stuck with needles or sharp objects containing HIV-infected blood. As for HIV-positive healthcare providers infecting their patients, there have only been six documented cases, all involving the same HIV-positive dentist in the 1980s.
Transmission via donated blood or blood clotting factors. However, this is now very rare in countries where blood is screened for HIV antibodies, including in the United States.

Since the beginning of the HIV/AIDS epidemic, new or potentially unknown routes of transmission have been thoroughly investigated by state and local health departments, in collaboration with the U.S. Centers for Disease Control and Prevention (CDC). To date, no additional routes of transmission have been recorded, despite a national system designed to detect unusual cases.

Sexual Transmission of HIV

In the United States, sexual contact is the most common route of HIV transmission. The CDC has published that of the 48,100 people who they estimated became infected in 2009, 57 percent were men who contracted HIV through sex with other men (MSM). The term MSM is important—and used quite a bit in this lesson—because many men who have sex with men do not necessarily identify themselves as "gay" or even "bisexual." HIV transmitted through sexual activity among heterosexuals accounted for 31 percent of new infections, with most of these cases among women infected by men. Injection drug users, in total, accounted for 12 percent of new infections, though about a quarter of those were MSM, so it isn't possible to know for sure whether those men were infected by sharing injection equipment or through sex.

Heterosexual intercourse is the most common mode of HIV transmission in many resource-poor countries. In Africa slightly more than 80 percent of infections are acquired heterosexually, while mother-to-child transmission and transfusions of contaminated blood account for the remaining infections. In Latin America, most infections are acquired by MSM and through misuse of injected drugs, but heterosexual transmission is rising. Heterosexual contact and injection of drugs are the main modes of HIV transmission in South and South East Asia.

The reason why sexual activity is a risk for HIV transmission is because it allows for the exchange of body fluids. Researchers have consistently found that HIV can be transmitted via blood, semen, and vaginal secretions. It is also true that HIV has been detected in saliva, tears, and urine. However, HIV in these fluids is only found in extremely low concentrations. What's more, there hasn't been a single case of HIV transmission through these fluids reported to the CDC.

Specific Sexual Practices: What are the Risks?

Studies have repeatedly demonstrated that certain sexual practices are associated with a higher risk of HIV transmission than others.

Vaginal Intercourse

Unprotected vaginal intercourse is the most common mode of HIV infection worldwide. At least five European and American studies have consistently demonstrated that male-to-female HIV transmission during vaginal intercourse is significantly more likely than female-to-male HIV transmission. In other words, HIV-positive men are much more likely to transmit the virus to HIV-negative women through vaginal intercourse than HIV-positive women are to HIV-negative men.

There are a few reasons for this. First, there are more men than women in the United States infected with HIV, meaning that it's much more likely for a female to have sex with an HIV-positive male than for a male to have sex with an HIV-positive female. Second, women have a much larger surface area of mucosal tissue—the lining of the vagina and cervix that can chafe easily and are rich in immune system cells that can be infected by HIV—than men do on their penises (though uncircumcised men do have mucosal tissue on their foreskins).

There have been a number of studies showing that men who are uncircumcised have a higher risk of becoming infected with HIV or transmitting the virus if they are already HIV positive. However, it is important to stress that men who are circumcised can still be infected (or transmit the virus) if condoms are not used for vaginal sex.

Men or women who have sexually transmitted infections (STIs) that cause sores on the anus, penis or vagina, such as genital herpes or syphilis, are more likely to spread the virus if they are HIV positive or to become infected with the virus if they are HIV negative.

To reduce the risk:
Correctly and consistently use latex or polyurethane condoms every time you have vaginal intercourse with a partner who is positive or whose HIV status you do not know.
Use a water-based or silicone-based lubricant with latex condoms. Lube keeps condoms gliding smoothly, reducing the risk of rips and tears. Lubrication also helps protect the vaginal wall from rips and tears, which can increase the risk of transmission.
Don't douche before engaging in vaginal intercourse. This can destroy the healthy bacteria in the mucosal lining of the vagina and can eliminate the vagina's natural lubrication.
Don't engage in unprotected vaginal intercourse during menstruation.

Anal Intercourse

Being the receptive partner—the "bottom"—during unprotected anal intercourse is linked to a high risk of HIV infection. The reason for this is that HIV-infected semen can come into contact with fragile and porous tissue in the anus that can be damaged easily during anal intercourse. And the risk of HIV transmission isn't necessarily reduced if the "top" pulls out before ejaculation—studies have demonstrated that pre-ejaculate (pre-cum) can contain high amounts of HIV and can result in transmission during anal intercourse.

It's important to note that both MSM and heterosexuals have anal intercourse. Many heterosexuals report that it is a pleasurable form of intimacy that eliminates the risk of pregnancy. However, it is still associated with a high risk of HIV infection, if condoms are not used and the insertive partner's HIV status is either positive or not known.

We know that men can be infected with HIV through vaginal intercourse—an activity in which they are the insertive partner. Based on this knowledge, it is also believed that the insertive partner during unprotected anal intercourse can also be infected with HIV. Studies, using mathematical estimates, suggest that unprotected insertive anal sex is roughly four to 14 times less risky than unprotected receptive anal sex. However, experts still believe that the risk for transmission is noteworthy.

To reduce the risk:
Correctly and consistently use latex or polyurethane condoms every time you have vaginal intercourse with a partner who is positive or whose HIV status you do not know.
Use a water-based or silicone-based lubricant with latex condoms. Lube keeps condoms gliding smoothly, reducing the risk of rips and tears. Lubrication also helps protect the vaginal wall from rips and tears, which can increase the risk of transmission.
Don't douche before engaging in anal intercourse. This can destroy the healthy bacteria in the mucosal lining of the anus and can eliminate the anus's natural lubrication.

Penile-Oral Sex

Of the different sex acts, the one that often causes the greatest amount of confusion in terms of risk—and raises the greatest number of questions—is penile-oral sex. The fact is, most experts agree that fellatio, sometimes referred to as "blow jobs," is not an efficient route of HIV transmission. However, this does not mean that it cannot happen.

Because unprotected fellatio can mean that body fluids from one person can (and do) come into contact with the mucosal tissues or open cuts, sores, or breaks in the skin of another person, there is a "theoretical risk" of HIV transmission. "Theoretical risk" means that passing an infection from one person to another is considered possible, even though there haven't been any (or only a few) documented cases. This term can be used to differentiate from documented risks. Having unprotected receptive anal or vaginal intercourse with an HIV-positive partner is a documented risk, as they have been shown in numerous studies to be an independent risk factor for HIV infection. Having unprotected oral sex is a theoretical risk, as it is considered possible, but has never been shown to be an independent risk factor for HIV infection.

There have been a number of studies that have closely followed MSM and heterosexual couples, in which one partner was HIV positive and the other partner was HIV negative. In all of the studies, couples that used condoms consistently and correctly during every experience of vaginal or anal sex—but didn't use condoms during oral sex—did not see HIV spread from the HIV positive partner to the HIV negative partner.

There have been three case reports and a few studies suggesting that some people have been infected with HIV as a result of unprotected oral sex. However, these case reports and studies all involved MSM—men who were the receptive partners (the person doing the "sucking") during unprotected oral sex with another HIV-positive man. There haven't been any case reports or studies documenting HIV infection among female receptive partners during unprotected oral sex. Even more importantly, there hasn't been a single documented case of HIV transmission to an insertive partner (the person being "sucked") during unprotected oral sex, either among MSM or heterosexuals.

Is insertive oral sex a possible route of HIV transmission? Yes. But is it a documented risk? Absolutely not.

To reduce the risk:
Don't get cum in your mouth. Also consider using an unlubricated (possibly flavored) condom every time you have oral sex with a partner who is positive or whose HIV status you do not know.
Avoid brushing or flossing your teeth immediately before oral sex. This reduces the risk of cuts, tears or abrasions in the mouth that can serve as an entry way for HIV.

Oral-Vaginal Sex

Like the study of fellatio, evaluating the risk of unprotected oral-vaginal sex (cunnilingus) is difficult, given that most people surveyed in studies did not avoid other types of unsafe sexual activity. However, there have been case reports highlighting one case of female-to-female transmission of HIV via cunnilingus and another case of female-to-male transmission of HIV via cunnilingus. Both of these cases involved transmission from receptive partner (the one receiving oral sex) to the insertive partner (the one performing oral sex). There haven't been any documented cases of HIV transmission from the insertive partner to the receptive partner.

To reduce the risk:
Use a latex barrier—such as a natural rubber latex sheet, a dental dam or a cut-open condom that makes a square—between your mouth and the vagina. A latex barrier reduces the risk of blood or vaginal fluids entering your mouth. Plastic food wrap also can be used as a barrier.

Oral-Anal Sex

Oral-anal sex is often referred to as analingus. Analingus, or "rimming," is not considered to be an independent risk factor for HIV. However, it has been shown to be a route of transmission for hepatitis A and B, as well as parasitic infections like giardiasis and amebiasis.

To reduce the risk:
Use a latex barrier—such as a natural rubber latex sheet, a dental dam or a cut-open condom that makes a square—between your mouth and the anus. A latex barrier reduces the risk of blood or anal fluids entering your mouth. Plastic food wrap also can be used as a barrier.

Digital-Anal or Digital-Vaginal Sex

Digital-anal or digital-vaginal sex is the clinical term for "fingering" either the anus or the female genitals (including the vagina). While it is theoretically possible that someone who has an open cut or fresh abrasion on his or her finger or hand can be infected with HIV if coming into contact with blood in the anus or vagina or vaginal secretions, there has never been a documented case of HIV transmission via fingering.

How can the sexual transmission be prevented?

"Safer sex" is a term that has been in existence since the very early years of the HIV epidemic. It can mean many things: Abstinence from sex, limiting the number of sexual partners, curtailing alcohol and drug use (which can impair judgement), avoiding bodily fluid exchange, using prophylactics (such as male or female condoms, which are discussed in this section), and the use of HIV medications to prevent transmission of the virus. In fact, safer sex often involves a combination of these approaches.

Male Condoms

Several studies have demonstrated that male condoms made of either latex or polyurethane are effective barriers against HIV. The theory behind using condoms is clear: they cover the penis and provide an effective barrier to exposure to secretions such as semen and vaginal fluids, thereby blocking sexual transmission of HIV infection.

Laboratory studies have been conducted to support this theory. These studies involved placing a solution containing HIV inside the condoms. No leakage of HIV across the latex or polyurethane condoms was demonstrated. Similar studies have also demonstrated that other common sexually transmitted viral infections, such as herpes simplex virus (HSV) and hepatitis B virus (HBV), are also prevented with the use of these two types of condoms.

Condoms made of "natural" materials—such as lambskin—are not a consistently effective barrier against many viruses. In one laboratory study, HIV was found to pass through microscopic holes in lambskin condoms. Studies involving HSV and HBV reported similar results.

There have been a number of epidemiological studies—studies that are conducted in real-life setting, where one partner is infected with HIV and the other partner is not—that have demonstrated consistent use of latex (or polyurethane) condoms provide a high degree or protection against HIV. However, the key to effective protection is consistent and correct use of condoms.

Incorrect use of condoms can increase the risk of condom slippage or breakage, which diminishes their protective effect. Inconsistent use—for example, failure to use condoms with every act of vaginal or anal intercourse—can lead to HIV transmission.

A word about polyurethane condoms: They are an effective alternative to latex condoms, especially for people with an allergy to latex. There have been at least six epidemiological studies of polyurethane condoms. Three of the studies found that that slippage and breakage occurs equally (and rarely) with both latex and polyurethane condoms. The three other studies found that polyurethane condoms are more likely to break than latex condoms (with one of the studies also demonstrating that polyurethane condoms are more likely to slip than latex condoms). Still, if used consistently and correctly, they are considered to be a highly effective barrier against the sexual transmission of HIV.

Female Condoms

The female condom, approved in 1993 for use in the United States, is a polyurethane pouch with flexible polyurethane rings and each end. It is inserted deep in the vagina, much like a diaphragm. The ring at the closed end holds the pouch in the vagina. The ring at the open end stays outside the vulva (vaginal opening). If inserted properly, it lines the vagina and the cervix, which helps to prevent pregnancy, along with HIV and other sexually transmitted infections.

While female condoms are not approved for use during anal intercourse, some MSM have reporting using them for anal sex. However, at least one study has reported problems for the receptive partner using the female condom, including difficulty inserting the condom, discomfort, and rectal bleeding (removing the inner ring may alleviate some of the problems experienced during anal insertion and removal). Several studies have also indicated that female condoms are not as effective as male condoms, largely because of the difficulty in using them correctly.

How are condoms used correctly?

Contrary to popular opinion, it's not only the sexually inexperienced who aren't familiar with how to use a condom effectively. Whether you're just starting to have sex—or have been going at it for years—a little information may be useful and important.

  • Men: Practice using male condoms while masturbating. MSM hoping to use female condoms for receptive anal sex are also encouraged to practice inserting and removing the condom before using it during intercourse.
  • Women: Practice using male condoms on penis-shaped objects, including ketchup bottles or bananas. Practicing the insertion and removal of female condoms, before they are used during vaginal intercourse, is also recommended.

Remember that the condom must be on the penis before it is inserted into the vagina or anus. The same holds true for female condoms—they must be inserted properly before intercourse begins.

Male condoms should be used only once. Use a new male condom for each episode of intercourse. One study has suggested that female condoms can be reused up to five times, provided that they are disinfected with bleach and water. However, experts caution that the safest way to use female condoms is to use them only once and then discard them.

Here are the key points that always need to be remembered when using male condoms:

Putting on a condom:

Condoms are individually sealed in aluminum or plastic wrapping. Be careful not to tear the condom while unsealing it. Never use a condom that is torn or seems brittle or stiff, past its expiration date, or exposed to extreme heat or cold.
If not circumcised, pull back the foreskin before rolling on the condom.
Leave a half-inch space at the tip of the condom to collect semen. Pinch the air out of the tip with one hand while unrolling the condom over the penis with the other hand.
Roll the condom down to the base of the penis.
Smooth out any air bubbles and lubricate the outside of the condom generously.
Use only one condom at a time. Using two condoms at a time, including two male condoms or a male and a female condom, can increase friction and lead to breakage.

Taking the condom off:

Be sure to pull out of the vagina or anus before the penis goes soft.
Clasp the condom against the base of the penis while pulling out.
Throw the condom away immediately.
Wash the penis with soap and water before post-sex intimacy.

If the condom breaks during intercourse:

Pull out quickly and replace it. Men should be able to tell if a condom breaks during intercourse. To learn what it feels like, men should purposely break a condom while masturbating.
If semen leaks out during intercourse and the insertive partner is HIV positive (or his HIV status is not known), contact a healthcare provider or hospital emergency room to discuss the risk and the possibility of post-exposure prophylaxis (PEP).

PEP involves a 28-day course of antiretroviral (ARV) drugs that needs to be started within 72 hours of possible exposure to the virus. Generally speaking, only people who have had a high-risk situation (e.g., condomless receptive anal or vaginal intercourse with someone known to be HIV positive) are considered to be good candidates for PEP.

A word about lubricants

Only water-based and silicone-based lubricants should be used with latex condoms. K-Y Jelly, Wet, and Astroglide are three examples of water-based lubricants that can be used with latex condoms. Examples of silicone-based lubricants include Millennium ID and Eros Bodyglide

Never use oil-based lubricants, including hand or body lotion, baby oil, vegetable oil or shortening, massage oil, mineral oil, or petroleum jelly (e.g., Vaseline). Oil-based lubricants can damage latex and cause latex condoms to tear more easily.

Some pre-lubricated condoms and separately sold lubricants contain a chemical called nonoxynol-9. While nonoxynol-9 has been shown to kill sperm (and potentially reduce the risk of pregnancy) and various sexually transmitted infections, some men and women are allergic to this chemical. This can cause irritation inside the vagina and anus, which can increase the risk of HIV transmission if the condom breaks.

Antiretroviral Drugs—PEP, PrEP and Treatment as Prevention

HIV drugs (antiretrovirals, or ARVs) are gaining considerable attention, not only for their ability to prolong disease-free survival for those living with HIV, but also for the role they may play in preventing transmission of the virus:

Post-exposure prophylaxis (PEP): PEP involves taking a short course of ARV drugs, usually for a month, after a high-risk exposure. To be most effective, PEP should be started immediately after possible exposure, waiting no more than 72 hours.

If you suspect a high-risk exposure to HIV—semen leaking out of a condom during intercourse with an HIV-positive insertive partner; receptive anal sex without a condom with a partner who is either HIV positive or whose status you do not know or you have shared drug-injection works with someone who is either HIV positive or whose status you do not know—contact your health care provider or local hospital emergency room as soon as possible.

Pre-exposure prophylaxis (PrEP): PrEP involves having an uninfected person take ARV drugs—usually Truvada (tenofovir plus emtricitabine)—before, during and after possible high-risk exposures to reduce the risk of becoming infected with HIV. Based on the results of clinical trials completed to date, the U.S. Food and Drug Administration (FDA) has approved Truvada as PrEP, with the requirement that it be used every day, even during periods of minimal or low-risk sexual activity. Future studies may explore intermittent dosing strategies (e.g., using PrEP only during periods of high-risk sexual or drug-using activity). 

Before providers prescribe PrEP, the U.S. Centers for Disease Control and Prevention (CDC) recommend a thorough assessment of a person’s HIV risk behaviors. An HIV test (a more sensitive test to determine a recent infection may be used in those with symptoms) and tests for sexual transmitted infections (STIs) are recommended. So, too, are tests for kidney function—the tenofovir in Truvada is associated with kidney toxicity—and hepatitis B virus (HBV) infection, given that Truvada is also active against HBV and must be used cautiously.

Providers are cautioned to prescribe no more than a 90-day supply of PrEP and to offer extensive HIV risk-reduction counseling, adherence counseling and condoms.

Follow-up is recommended every two to three months to test again for HIV. Further assessments are also recommended at this time for adherence and continued HIV risk behavior, and to provide ongoing support and counseling for these. Kidney function testing is again recommended three months after a person first starts PrEP and yearly thereafter. Tests for common STIs are also recommended every six months, even if a person has no symptoms.

The CDC also recommends that before people discontinue PrEP—whether because of safety concerns, a positive HIV test result, or a person requests to stop treatment—their providers should link them to HIV care (if a person has become infected) or ongoing HIV risk-reduction counseling and support. For people who have HBV, their providers should also discuss whether to continue treatment as a means to control HBV infection.

Treatment-as-prevention: Whereas PrEP focuses on prescribing ARVs to people who aren't infected with HIV to help them remain free of the virus, treatment-as-prevention (TasP) involves prescribing ARVs to those who are infected with HIV in order to reduce the amount of virus in their blood (and genital fluids) so that they are less likely to infect others.

One clinical trial, initially reported at a conference in July 2011, suggested TasP may be effective. The study, HIV Prevention Trials Network (HPTN 052) demonstrated that the use of ARVs by HIV-positive heterosexual men and women cut the chance that their HIV negative partner would become infected by roughly 96 percent.

The U.S. Department of Health and Human Services now recommends ARV treatment for all people living with HIV, regardless of their CD4 cell count, based in part on the findings of HPTN 052 and other studies. This recommendation has been somewhat controversial, as the studies used to support it (including HPTN 052) primarily involved heterosexual couples in long-term monogamous relationships (TasP has not been studied in populations of men who have sex with men or injection drug users) and do not account for the variables in real-world situations (e.g., HIV-positive individuals with multiple partners, individuals engaging in unprotected anal sex, people on ARV treatment with drug resistance and detectable viral loads, etc.). These lingering questions have prompted additional research to explore not only the personal benefits of treatmentAIDS-free survival for the person infected with HIVbut also the public health implications of getting all HIV-positive people, especially those who are unaware of their status, in to care and on treatment to reduce the ongoing spread of HIV.

Vaginal and Rectal Microbicides: Microbicides are an emerging technology designed to allow at-risk HIV-negative women and men to protect themselves from HIV. A microbicide has not yet been approved for this purpose.

In July 2010, a clinical trial (CAPRISA 004) found that a microbicide gel containing the ARV Viread (tenofovir) demonstrated a 39 percent level of efficacy at preventing HIV infection when applied vaginally within 12 hours before and within 12 hours after sex. In November 2011, however, an interim review of data from another clinical trial (the VOICE study) revealed that the tenofovir gel was no more effective than a placebo gel at preventing HIV infection among females who used the microbicide once daily, regardless of the timing or frequency of sexual acts.

Microbicide gels, applied vaginally and anally, continue to be explored in clinical trials. Researchers are also beginning studies of silicone rings containing ARVs that can be inserted vaginally and replaced on a monthly basis, making the microbicide easier to use (and potentially more effective).

When In Doubt, Test!

While we know that some types of sexual activities are much lower risk than others (with some activities only being a theoretically risk, not a proven risk) and the proven effectiveness of condoms, there are no guarantees. The fact is, it is very difficult to be 100% certain that you did not engage in any risk behavior, especially if you have any sort of unprotected sexual activity with a person known to be HIV positive or you do not know what his or her HIV status is. Abstinence—or sexually activity only in a monogamous relationship in which both partners are HIV negative—is the only way to eliminate the risk of HIV infection.

The only way to know for sure whether you have been infected, even after an activity that is considered to be very low risk for HIV transmission, is to get tested. Also, if you've been infected with another STD, getting tested for HIV too is a really good idea. For more information about testing, please read's comprehensive lesson on this subject:

Lesson Am I Infected? (A Complete Guide to Testing for HIV)

Last Revised: August 17, 2012

This content is written by the POZ and AIDSmeds editorial team. For more information, please visit our "About Us" page.

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