STIs in males are caused by bacteria, viruses, or parasites that enter the body during sexual contact. The specific pathogen depends on the infection, but the common thread is transmission through vaginal, anal, or oral sex with an infected partner. In 2024, over 900,000 cases of chlamydia and gonorrhea alone were reported among men in the United States.
Bacterial STIs
Bacteria cause several of the most common STIs in men. Chlamydia is the most frequently reported, with roughly 564,000 cases diagnosed in U.S. males in 2024. Gonorrhea followed with about 342,000 cases. Both infections typically target the urethra, the tube that runs through the penis, causing burning during urination and discharge. But many men have no symptoms at all, which means they can unknowingly pass the infection to partners.
Syphilis is caused by a different type of bacterium and progresses through stages if untreated, starting with a painless sore at the site of contact. Provisional 2024 data show over 29,000 cases of early-stage syphilis in men. A less well-known bacterial infection, Mycoplasma genitalium, also causes urethral inflammation and is increasingly recognized as a significant cause of persistent or recurring symptoms.
These bacteria are spread through direct contact with infected mucous membranes or fluids during vaginal, anal, or oral sex. Gonorrhea-related bacteria can also colonize the throat, meaning oral sex is a real transmission route in both directions.
Viral STIs
Viruses cause some of the most persistent STIs because, unlike bacterial infections, they often can’t be fully cleared from the body. The major viral STIs affecting men include HPV (human papillomavirus), herpes (HSV), HIV, and hepatitis B.
HPV spreads through close skin-to-skin contact during sex, not just through fluids. This makes it especially easy to transmit. Most men with HPV never develop symptoms or health problems, but the virus can cause genital warts and, in some cases, cancers of the throat, penis, or anus. A person with HPV can pass it to a partner even with no visible signs of infection.
Herpes (HSV) similarly spreads through skin contact, often from areas not covered by a condom. It causes recurring outbreaks of painful sores, though many carriers shed the virus between outbreaks without knowing it. Viruses like Epstein-Barr and adenovirus can also infect the urethra, particularly through oral sex.
HIV is primarily a concern during anal and vaginal intercourse. The risk varies dramatically by the type of contact. Receptive anal sex carries the highest per-act risk at roughly 1 in 72 exposures (without condoms or preventive medication). Insertive vaginal sex carries a much lower per-act risk of about 1 in 2,500. These numbers assume no condoms, no PrEP, and no treatment suppressing the infected partner’s viral load.
Parasitic STIs
Trichomoniasis is caused by a microscopic parasite that, in men, typically infects the inside of the urethra. The parasite spreads from a penis to a vagina or from a vagina to a penis during sex. Men with trichomoniasis often have no symptoms, or they may notice mild irritation, burning after urination, or slight discharge. Like bacterial STIs, people with trichomoniasis can pass it to partners even without symptoms. Pubic lice and scabies also spread through close sexual contact, though they infect the skin rather than the urogenital tract.
How Pathogens Enter the Male Body
The urethra is the primary entry point for most STIs in men. During vaginal or anal intercourse, bacteria and viruses in a partner’s genital or rectal fluids come into direct contact with the urethral opening. The thin, moist lining of the urethra is vulnerable to infection in a way that external skin generally is not.
Oral sex creates a different set of risks. Bacteria that normally live in the throat, including certain strains related to meningitis-causing bacteria, can be transmitted to the penis during oral contact. Conversely, gonorrhea and herpes can travel from the genitals to the throat. Insertive anal intercourse can expose the urethra to gut bacteria, which occasionally cause urethral infections.
For skin-to-skin viruses like HPV and herpes, the pathogen doesn’t need to enter the urethra at all. Contact with infected skin anywhere in the genital region is enough. This is why condoms reduce but don’t eliminate the risk of these infections.
Non-Sexual Transmission Routes
While sexual contact is the primary cause of STIs, a few can spread through other means. HIV and hepatitis B can transmit through shared needles, contaminated blood products, or from mother to child during birth or breastfeeding. Syphilis can theoretically spread through direct contact with an open sore, though non-sexual transmission in adults is extremely rare. HIV transmission through tattooing with contaminated equipment has been reported but is uncommon in regulated settings.
Factors That Increase Risk
Several factors make some men more likely to contract an STI. Having multiple sexual partners increases the statistical probability of encountering an infected person. Inconsistent condom use is a major factor, particularly for bacterial and parasitic infections where condoms are highly effective. Anal intercourse carries higher transmission risk for several STIs compared to vaginal sex, partly because the rectal lining is thinner and more prone to small tears that allow pathogens through.
Circumcision status plays a measurable role. Voluntary male circumcision is associated with roughly a 60% reduction in the risk of female-to-male HIV transmission, based on large-scale studies in sub-Saharan Africa. The foreskin contains a high density of immune cells that HIV targets, and the moist environment under the foreskin may help pathogens survive longer.
Having one STI also increases vulnerability to others. Genital sores from herpes or syphilis create breaks in the skin that make HIV transmission more likely. Inflammation from chlamydia or gonorrhea similarly increases susceptibility. This is one reason early detection and treatment of any STI matters, even when symptoms seem mild or manageable.