Young people between 15 and 24 account for half of all new sexually transmitted infections in the United States, despite making up only about 25% of the sexually active population. But age is just one piece of the picture. STI risk is shaped by biology, behavior, social circumstances, and geography, and several groups face disproportionately high rates for overlapping reasons.
Young Adults Ages 15 to 24
This age group carries the single largest share of new STI cases. Several factors converge to explain why. Younger people are more likely to have multiple sequential partners, less likely to use barrier protection consistently, and less likely to get tested regularly. Adolescent girls face an additional biological vulnerability: the cells lining the cervix are more exposed during the teenage years (a normal developmental feature called cervical ectopy), which makes it easier for infections like chlamydia and gonorrhea to take hold.
Access matters too. Many young people are on a parent’s insurance and may avoid seeking testing out of concerns about privacy. Others age out of pediatric care without establishing a relationship with an adult provider, creating a gap in routine screening.
Women and the Biology of Transmission
STIs transmit more efficiently from men to women than the other way around. The vaginal lining is thinner and more delicate than penile skin, making it easier for infectious agents to penetrate. The exposed surface area is also larger, which means more tissue is in contact with potentially infectious fluids during sex.
This biological asymmetry shows up across nearly every STI. Women are more likely to contract chlamydia, gonorrhea, and herpes from a single sexual encounter than men are. Compounding the problem, many STIs produce fewer noticeable symptoms in women, so infections can go undetected and untreated for longer, increasing the window for complications and onward transmission.
Men Who Have Sex With Men
Men who have sex with men (MSM) experience some of the highest STI rates of any population group. Pooled research data show gonorrhea incidence rates among high-risk MSM ranging from about 13 to 43 new infections per 100 people per year, and chlamydia rates in a similar range. Syphilis incidence in this group runs around 9 new cases per 100 people per year.
Several factors drive these numbers. Anal tissue is more susceptible to micro-tears during sex, creating direct entry points for bacteria and viruses. Dense sexual networks in urban areas can accelerate transmission. And stigma still discourages some men from disclosing their sexual history to healthcare providers, which can delay testing and treatment.
One promising development: a preventive antibiotic strategy called doxy-PEP, in which a dose of doxycycline is taken within 72 hours of condomless sex, reduced chlamydia infections by 88%, syphilis by 87%, and gonorrhea by 55% in a clinical trial among MSM and transgender women. The CDC now recommends this approach for MSM and transgender women who have had a bacterial STI in the past 12 months.
People Who Use Drugs or Alcohol Before Sex
Substance use is one of the strongest behavioral predictors of STI risk. In a nationally representative U.S. survey, young adults who used illicit drugs in the past year were roughly three times more likely to report an STI than those who did not. That association held even after researchers controlled for age, race, marital status, and other factors.
The connection works through multiple pathways. Drugs and alcohol impair judgment and lower inhibitions, making condomless sex and sex with unfamiliar partners more likely. Methamphetamine, ketamine, and inhalants have been specifically linked to higher rates of STI and HIV co-infection among MSM in urban settings. For people who inject drugs, sharing needles adds the risk of blood-borne infections like HIV and hepatitis C on top of sexually transmitted ones. And in some cases, sex is exchanged directly for drugs, further increasing exposure.
People Living in Poverty or Without Insurance
STI rates track closely with socioeconomic disadvantage. Lower income, high unemployment, and lack of health insurance all correlate with higher rates of chlamydia, gonorrhea, syphilis, and HIV. The reasons are practical: fewer nearby clinics, longer wait times, inability to pay for visits or prescriptions, and less access to preventive tools like condoms or PrEP.
States that have not expanded Medicaid coverage show significantly higher STI rates at the county level. When people with infections can’t access treatment, they remain infectious longer, which sustains transmission across entire communities. High-poverty neighborhoods in large metro areas see elevated rates of nearly every STI, driven by a combination of limited healthcare resources and the downstream effects of economic instability on sexual behavior and partner networks.
Geographic Hotspots in the U.S.
Where you live affects your risk. Mississippi and Louisiana consistently rank among the top states for gonorrhea, chlamydia, and syphilis. South Dakota leads the nation in both primary/secondary syphilis and congenital syphilis rates. Alaska, Alabama, and Georgia also appear repeatedly in the top five across different STI categories. Southern states as a whole carry a disproportionate burden, reflecting the region’s higher poverty rates, larger uninsured populations, and fewer sexual health clinics per capita.
People With an Existing STI
Having one STI makes it substantially easier to acquire another, especially HIV. Genital ulcers caused by infections like herpes or syphilis increase the per-act risk of contracting HIV by roughly five times. The ulcers break down the skin and mucosal barriers that normally block viral entry, and they attract exactly the type of immune cells that HIV targets.
Non-ulcerative STIs like chlamydia and gonorrhea also increase HIV susceptibility by triggering inflammation in the genital tract. This creates a cycle: untreated STIs raise HIV risk, and HIV weakens the immune system in ways that make future STIs harder to clear.
Recommended Testing Intervals
The CDC tailors screening recommendations to risk level. Sexually active women under 25 should be tested for chlamydia and gonorrhea annually. MSM at increased risk, including those on HIV prevention medication (PrEP) or those with multiple partners, should be screened for chlamydia, gonorrhea, and syphilis every 3 to 6 months. The same 3-to-6-month interval applies to HIV testing for MSM at elevated risk.
Pregnant women get tested at the first prenatal visit, with repeat testing in the third trimester for those under 25 or with risk factors like new partners, substance use, or living in a high-prevalence area. Anyone diagnosed with an STI during pregnancy should be retested within 3 months. For people living with HIV, annual screening for common STIs is the baseline, with more frequent testing based on individual behavior.