How Do Men Get HIV or AIDS? Transmission and Risk

Men acquire HIV almost exclusively through unprotected sexual contact or through blood-to-blood exposure such as sharing injection equipment. Sexual transmission accounts for the vast majority of new infections in men worldwide, with the specific risk depending heavily on the type of sex act, the role played during that act, and a cluster of biological factors most men never think about. The picture is more layered than “use a condom,” and understanding those layers can change how you assess your own risk.

Which Sexual Acts Carry the Most Risk

Not all sex carries equal HIV risk. Anal intercourse is consistently the highest-risk sexual act, followed by vaginal intercourse, with oral sex presenting a much lower risk. Within those categories, being the receptive partner carries substantially more risk than being the insertive partner. Per-act risk estimates for unprotected receptive anal intercourse range from about 0.5% to 3.4%, while unprotected insertive anal intercourse carries a per-act risk in the range of 0.06% to 0.16%.¹ Those numbers sound small in isolation, but they compound with repeated exposures and become dramatically larger over the course of a sexual partnership. A systematic review estimated the per-partner risk for unprotected receptive anal intercourse at roughly 40%.²

For men who have sex with men, the role you take during anal sex is one of the strongest predictors of infection. A meta-analysis of global data found that men who engaged exclusively in receptive anal sex were about six times more likely to acquire HIV than men who engaged exclusively in insertive anal sex.³ Men who practiced both roles had similarly elevated risk. This difference is driven by biology: the lining of the rectum is a single cell layer thick in places and rich in immune cells that HIV targets, making it far more vulnerable to infection than the skin of the penis.

Vaginal intercourse poses a real but lower per-act risk for the insertive male partner. Early studies from Africa found that about 8% of men seroconverted after contact with HIV-positive female sex workers, but that figure was heavily shaped by the presence of other sexually transmitted infections and circumcision status.⁴ The risk difference between anal and vaginal routes matters for heterosexual men too: the per-act risk of anal intercourse does not appear to differ between heterosexual and same-sex couples, so heterosexual men who practice anal sex face the same biological risk profile as gay men for that act.¹

Oral sex is frequently described as “low risk” rather than “no risk.” It can transmit other sexually transmitted infections, and while documented cases of HIV transmission through oral sex exist, they are rare enough that per-act risk estimates are difficult to calculate reliably.

How Other Infections Amplify the Risk

Having another sexually transmitted infection at the time of exposure is one of the most powerful amplifiers of HIV transmission risk. The relationship works in both directions: an existing STI in the HIV-negative man makes him more vulnerable, and an STI in the HIV-positive partner makes them more infectious. Across a wide range of infections, observational evidence consistently shows a strong link between STIs and increased HIV acquisition.⁵

Genital ulcer diseases like herpes (HSV-2) and syphilis are especially dangerous in this regard. Ulcers break the skin or mucosal barrier, giving HIV a direct route into the bloodstream. But the mechanism goes deeper than just open sores. Tissue affected by HSV-2 or syphilis contains higher concentrations of the specific immune cells that HIV latches onto, effectively rolling out a welcome mat for the virus.⁶ One cohort study of men who have sex with men in Bangkok found that acquiring herpes more than doubled the risk of subsequently acquiring HIV.⁷

The STI effect applies to the female partner’s health as well. A prospective study of African couples found that men whose HIV-positive female partners had bacterial vaginosis faced more than three times the risk of acquiring HIV compared to men whose partners had normal vaginal flora, even after controlling for viral load, circumcision status, and condom use.⁸ This is a factor men rarely consider, because bacterial vaginosis often produces no visible symptoms in a female partner.

The Foreskin and Circumcision

Circumcision is one of the more surprising variables in male HIV risk. Large randomized trials have established that circumcision reduces heterosexual HIV acquisition in men by at least 60%.⁹ The World Health Organization has recommended voluntary male circumcision as an HIV prevention tool in high-prevalence settings since 2007.

The reason lies in the biology of the foreskin itself. The inner surface of the foreskin has a thinner outer layer than the rest of the penile skin and contains a higher density of the specific immune cells HIV needs to establish infection. Research has found that this inner surface harbors cells expressing the CD4 and CCR5 receptors that HIV uses as entry points, and the tissue shows signs of low-grade inflammation even in healthy men, with elevated levels of immune-signaling molecules that attract more vulnerable cells to the area.¹⁰ The warm, moist environment beneath an intact foreskin also supports a microbial community that can drive further immune activation and attract even more HIV target cells.¹¹

Removing the foreskin eliminates most of these mechanisms at once: the vulnerable inner surface is gone, the moist sub-preputial environment disappears, and the remaining penile skin keratinizes (toughens) over time. It is worth noting, though, that this protection is partial. Circumcision reduces risk, it does not eliminate it, and the evidence for a protective effect is strongest for heterosexual vaginal intercourse. The data on whether circumcision provides similar protection during anal intercourse is much less clear.

Blood-Borne Transmission

Sharing needles and syringes for injecting drugs is the primary non-sexual route of HIV transmission for men. The risk per shared needle is considerably higher than the risk per sexual act, because a contaminated needle delivers blood directly into the body. But research has shown that sharing other injection preparation equipment, like cookers, filters, and rinse water, also carries risk. Even when men avoid sharing the needle itself, frequently sharing preparation equipment can add up to meaningful cumulative exposure.¹²

Occupational needle-stick injuries in healthcare settings are another documented route, though the risk is small. A review of prospective studies calculated the risk of seroconversion following a percutaneous exposure (such as a hollow-bore needle stick) at about 0.33%, or roughly 3 in 1,000 exposures. The risk from splash exposure to mucous membranes was much lower, around 0.04%.¹³ Most documented occupational infections involved hollow-bore needle injuries during blood-drawing procedures.

Why the Infected Partner’s Viral Load Matters More Than Almost Anything

The amount of virus circulating in the HIV-positive partner’s blood and genital fluids is probably the single most important factor in whether transmission occurs during any given exposure. During the first weeks after someone acquires HIV, known as acute or primary infection, viral load spikes dramatically and the person becomes extraordinarily infectious. A meta-analysis found that the rate of transmission during this brief primary infection period was roughly nine to 37 times higher than during the long, stable phase that follows, depending on modeling assumptions.¹⁴ This matters because most people in acute infection do not yet know they are infected, and their genital secretions carry especially high viral concentrations.¹⁵

On the opposite end, effective antiretroviral treatment that suppresses viral load to undetectable levels essentially eliminates the risk of sexual transmission. Three large studies tracking thousands of serodiscordant couples (where one partner has HIV and the other does not) found zero linked transmissions when the HIV-positive partner maintained a viral load below 200 copies per milliliter.¹⁶ This is the foundation of the “Undetectable equals Untransmittable” (U=U) message. Population-level data from surveys in multiple countries has confirmed that people on successful treatment who maintain suppressed viral loads pose no meaningful transmission risk to sexual partners.¹⁷

Network Effects and Chemsex

Individual risk behavior only tells part of the story. The structure of sexual networks plays a surprisingly large role in who gets infected. Research on men who have sex with men has found that network characteristics, like how densely connected a group is and whether people tend to have overlapping (concurrent) sexual partnerships, can matter as much as or more than individual choices about condom use.¹⁸ When sexual partnerships overlap in time, a newly infected person can pass the virus along before they have any idea they are carrying it, especially during that high-viral-load acute phase.

This network effect helps explain a well-documented paradox in U.S. HIV epidemiology: Black men who have sex with men experience disproportionately high HIV rates despite reporting less frequent condomless sex than some other groups. The disparity is driven in part by high levels of racial homophily in sexual networks. Because HIV prevalence is already high among Black MSM, the probability of encountering an infected partner within a racially homophilous network is greater, regardless of individual behavior.¹⁸ Partner concurrency amplifies this further: among Black men in one study, concurrent partnerships were associated with about a threefold increase in transmission potential.¹⁹

Sexualized drug use, often called chemsex, represents another behavioral layer that increases risk in ways that go beyond the drugs themselves. Studies have found that chemsex is associated with roughly five to six times higher odds of condomless anal intercourse, and with significantly higher rates of bacterial STI diagnoses and hepatitis C.²⁰ Injecting drugs during sex, sometimes called “slamsex,” carries even higher risk associations. Research from Thailand found that chemsex users were more likely to report six or more sexual partners in a six-month period and less likely to use condoms.²¹ The combination of impaired judgment, more partners, more condomless sex, and sometimes needle sharing during injection creates a cluster of risk that is difficult to disentangle into neat individual factors.

PrEP and PEP as Prevention Tools

Pre-exposure prophylaxis (PrEP) is the biggest shift in HIV prevention for men in the last two decades. Daily oral PrEP with tenofovir/emtricitabine has been available since 2012, and event-driven (on-demand) dosing, where pills are taken around the time of sexual activity, has also shown powerful results. The ANRS IPERGAY trial found that on-demand PrEP reduced HIV incidence by about 97% compared to placebo among men who have sex with men.²²

Long-acting injectable cabotegravir, given as an injection every two months, has demonstrated even higher effectiveness than daily pills. In the HPTN 083 trial, injectable cabotegravir reduced HIV incidence among men who have sex with men and transgender women by about 69% compared to daily oral PrEP, which was already highly effective.²³ The advantage appears to come from removing the adherence challenge: you cannot forget to take a pill you never had to swallow. Extended follow-up confirmed these results, with HIV incidence in the injectable group remaining lower during the first unblinded year of the study.²⁴

Post-exposure prophylaxis (PEP) is the emergency-room counterpart to PrEP: a 28-day course of antiretroviral drugs started within 72 hours of a potential exposure. PEP is most familiar in occupational settings after needle-stick injuries, but it is available to anyone after a sexual exposure. The challenge is completion. One study found that only 60% of people prescribed PEP for non-occupational exposure finished the full 28-day regimen without interruption, and about 20% never returned for their first follow-up visit.²⁵ Side effects and the demanding dosing schedule likely play a role in dropouts, which makes PEP a valuable but imperfect safety net.

Rectal Douching and Mucosal Vulnerability

Rectal douching before anal sex is common among men who have sex with men, and it may increase HIV susceptibility in a way most people do not realize. A recent study found that men who douched had significantly elevated levels of a blood marker called lipopolysaccharide-binding protein, which signals that bacteria are crossing from the gut into the bloodstream. This suggests that douching disrupts the integrity of the rectal lining, the same barrier that normally provides at least some protection against HIV entry.²⁶ While douching is often framed purely as a hygiene choice, the biological evidence hints that it may be quietly increasing the very risk it has no bearing on in most people’s mental models of HIV transmission.

Genetic Resistance Is Real but Extremely Rare

A small fraction of men carry a genetic variation that provides partial or near-complete protection against HIV. The mutation, called CCR5-delta32, affects the CCR5 receptor that HIV uses to enter immune cells. People who inherit two copies of this mutation (one from each parent) have virtually no functional CCR5 receptors on their cells, making it extremely difficult for the most common strains of HIV to establish infection. A meta-analysis found that carrying two copies of the delta32 mutation reduced the odds of HIV infection by about 75%.²⁷

Having just one copy provides a more modest and complicated effect. Among people who have been repeatedly exposed to HIV but remain uninfected, carrying one copy of the delta32 mutation along with a specific promoter variation on the other copy of the CCR5 gene was associated with reduced CCR5 density on immune cells, which appeared to help resist sexual transmission.²⁸ The mutation is found predominantly in people of Northern European descent and is rare or absent in most African and Asian populations. No one should treat genetic resistance as a reason to forgo prevention: even among carriers, protection is not absolute, and strains of HIV that use a different receptor (CXCR4) can bypass the CCR5 block entirely.

What Does Not Transmit HIV

Misconceptions about HIV transmission persist despite decades of public health messaging. HIV is not spread through casual contact, saliva, sweat, tears, sharing food or drinks, toilet seats, swimming pools, or insect bites. The virus is fragile outside the body and does not survive in the environment for any meaningful length of time. Kissing, even deep kissing, does not transmit HIV unless both partners have significant open sores or bleeding gums and blood is exchanged, and even then the documented risk is essentially theoretical. These may sound like basic points, but surveys consistently show that a meaningful percentage of people still believe in casual transmission routes, and that stigma built on these misconceptions remains a barrier to testing and treatment. For men evaluating their own risk, the core question is straightforward: did a sexual or blood-to-blood exposure to an HIV-positive person with a detectable viral load occur? If not, the risk of acquiring HIV from everyday life is effectively zero.