Men get AIDS as the result of an HIV infection that has gone untreated for years. HIV itself is most commonly transmitted through anal or vaginal sex without protection, or by sharing needles and other drug injection equipment. Without treatment, HIV typically progresses to AIDS within about 10 years, though modern antiretroviral therapy can prevent that progression entirely.
Understanding the specific ways HIV enters the body, and how much risk each type of exposure actually carries, helps put the real picture into focus.
Sexual Transmission: The Most Common Route
The vast majority of HIV infections in men happen through sex. Both anal and vaginal intercourse can transmit the virus, but the level of risk varies dramatically depending on the type of sexual contact.
Receptive anal sex (bottoming) carries the highest per-act risk of any sexual activity: roughly 138 out of every 10,000 exposures to an HIV-positive partner, when no condoms, PrEP, or treatment are involved. That makes it far riskier than any other form of sex. Insertive anal sex (topping) carries a risk of about 11 per 10,000 acts. The rectal lining is thin and rich in immune cells that HIV targets, which is why receptive anal sex is so much more efficient at transmitting the virus.
Vaginal sex poses a lower but real risk. For a man in the insertive role, the estimated risk is about 4 per 10,000 acts. HIV can enter through the opening at the tip of the penis, through the foreskin, or through any small cuts or sores on the shaft. Research has shown that the virus can work its way past the outer skin layer of the penis and reach the immune cells underneath, particularly in the inner foreskin and the glans. This is one reason circumcision has been found to reduce the risk of female-to-male transmission by about 60%.
These per-act numbers may sound small, but risk accumulates with repeated exposures. A man having unprotected sex regularly with an HIV-positive partner faces substantially higher cumulative odds over weeks and months.
Why Existing STIs Raise the Risk
Having another sexually transmitted infection significantly increases the chance of picking up HIV during an exposure. Untreated syphilis, for example, raises the risk of both acquiring and transmitting HIV. Herpes sores and other STIs that cause open lesions or inflammation create more entry points for the virus, while also drawing more of the immune cells HIV targets to the genital area. Getting tested and treated for STIs is one of the most overlooked ways to lower HIV risk.
Sharing Needles and Injection Equipment
Sharing needles, syringes, or other drug injection equipment (like cookers) is the second most common way men get HIV. Used equipment can contain traces of infected blood, and injecting that blood directly into the body is an extremely efficient route of transmission. This applies to any type of injected drug, not just opioids or specific substances. The risk extends beyond the needle itself to any piece of equipment that might hold residual blood.
Less Common Transmission Routes
Occupational exposure, such as a needlestick injury in a healthcare setting, is possible but extremely rare. Only 58 confirmed cases of occupational HIV transmission to healthcare workers have ever been reported in the United States, and just one confirmed case has occurred since 1999. A man can also get HIV through receiving a blood transfusion with infected blood, though modern screening has made this nearly nonexistent in countries with robust blood supply testing.
HIV is not transmitted through saliva, sweat, casual contact, sharing food or drinks, or being near someone who is coughing or sneezing. The virus cannot survive long outside the body and requires specific conditions to spread.
How HIV Becomes AIDS
HIV and AIDS are not the same thing. HIV is the virus; AIDS is the most advanced stage of the infection. After initial infection, HIV slowly attacks the immune system over years, targeting a specific type of white blood cell. A person is diagnosed with AIDS when their count of these protective cells drops below 200 per cubic millimeter of blood, or when they develop certain serious infections that a healthy immune system would normally fight off.
Without any treatment, this process typically takes about 10 years, though it can happen faster in some people. With modern antiretroviral therapy, most people with HIV never develop AIDS at all. Treatment keeps the virus suppressed and the immune system intact, allowing a normal or near-normal lifespan.
Prevention Tools That Work
Condoms remain one of the most effective and accessible barriers against HIV transmission during sex. But several other tools now exist that dramatically change the math.
PrEP is a medication taken by HIV-negative people before potential exposure. When taken as prescribed, it reduces the risk of getting HIV from sex by about 99%. It’s available as a daily pill or, more recently, as a long-acting injection. PrEP is particularly relevant for men who have sex with men, men with HIV-positive partners, and anyone with ongoing risk factors.
Treatment as prevention is equally powerful. A person living with HIV who takes antiretroviral therapy and maintains an undetectable viral load has zero risk of transmitting the virus to sexual partners. This principle, known as Undetectable = Untransmittable (U=U), is backed by large-scale studies and endorsed by every major health organization. If a man’s partner is HIV-positive and on effective treatment, the sexual transmission risk drops to zero.
PEP is an emergency option for after a potential exposure. It involves a 28-day course of antiretroviral medication that must be started within 72 hours of exposure to be effective. The sooner it’s started, the better it works. PEP is available through emergency rooms and many clinics.
Between condoms, PrEP, treatment as prevention, and PEP, men today have more options to prevent HIV than at any point in the epidemic’s history. The key is knowing which tools are relevant and using them consistently.