The Most Common Sexually Transmitted Diseases and How They Spread

Published: July 2025 | Last updated: May 2026

Sexually transmitted infections sit closer to ordinary life than headlines suggest. The CDC reports more than 2.4 million combined cases of chlamydia, gonorrhea, and syphilis each year in the United States, and the World Health Organization estimates over one million curable STIs are acquired every day worldwide among people aged 15 to 49. Eight infections account for the bulk of what providers screen for. Most spread through routes that surprise people, and most cause no symptoms in the people carrying them.

This guide walks through each of those eight: how it spreads, how often it shows up silently, and what testing or vaccination looks like. The aim is plain information you can use to decide when to test, what to ask a clinician, and how to talk with a partner without panic.

The eight most common STIs at a glance

Public-health agencies group the most common sexually transmitted infections by what causes them and how they behave in the body. The table below summarizes the eight infections covered here, including how each is transmitted, whether it can be cured, and whether a vaccine exists. Detailed sections follow.

HPV: the silent virus most adults will encounter

Human papillomavirus is the most common STI in the world. The CDC estimates that nearly all sexually active adults who don't get vaccinated will be infected with at least one HPV strain at some point. There are over 100 strains in total; about 40 affect the genital area. Most infections clear within two years thanks to your immune system. The trouble comes when high-risk strains persist, because they can lead to cervical, anal, throat, or penile cancers years or decades later.

HPV spreads through direct skin-to-skin contact during vaginal, anal, or oral sex. No fluid exchange is required. Most people never know they're carrying it because it usually causes no symptoms. Some strains cause genital warts; others cause cellular changes detected only on a Pap smear, anal cytology, or HPV DNA test.

Once exposed, no antiviral treatment exists for the virus itself. The body either clears it or the immune system holds it in check while clinicians monitor for cellular changes through routine cervical screening and follow-up.

Chlamydia and gonorrhea: curable, but easy to miss

Chlamydia is the most commonly reported bacterial STI in the U.S. CDC surveillance counts over 1.6 million reported chlamydia cases annually, with gonorrhea ranking second at over 600,000 reported cases. Both can pass during oral, anal, or vaginal sex through semen, vaginal fluid, or infected mucosal tissue. Manual sex with infected fluids can also transmit, though less commonly.

The trouble with both: they often cause no symptoms. The CDC estimates roughly 70% of women and 50% of men with chlamydia notice nothing at all. Untreated, both bacteria can ascend to the upper reproductive tract and cause pelvic inflammatory disease in women or epididymitis in men. Long-term consequences include infertility, chronic pelvic pain, ectopic pregnancy risk, and increased HIV susceptibility from the local inflammation.

A short course of antibiotics cures most cases. Current CDC treatment guidelines recommend doxycycline for chlamydia and a ceftriaxone injection for gonorrhea. Reinfection is common when partners aren't treated together, so testing of partners and expedited partner therapy are standard practice. Antibiotic-resistant gonorrhea is a growing concern, with strains showing reduced susceptibility to several drug classes; the CDC updates the STI Treatment Guidelines as resistance patterns shift, which is why retesting after treatment is recommended for most cases.

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Self-collected swab covers the two most commonly reported bacterial STIs in one rapid lateral-flow kit. Useful for at-home screening from roughly 1 to 2 weeks after a possible exposure, with a 15-minute readout. A positive result is worth confirming with a lab NAAT.

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Syphilis: the old infection making a comeback

Syphilis cases in the U.S. have climbed sharply over the past decade. CDC surveillance reported over 200,000 total cases in 2022, including a continued rise in congenital syphilis (mother-to-baby transmission during pregnancy or birth). The bacterium Treponema pallidum spreads through direct contact with a syphilitic sore (called a chancre), which usually develops on the genitals, anus, mouth, or lips. Because the chancre is painless and may sit hidden inside the vagina, rectum, or mouth, many people pass syphilis on without ever noticing they have it.

A single penicillin G injection cures early-stage syphilis, but earlier treatment prevents the worst outcomes. Routine screening is recommended for everyone who is pregnant (at least once, ideally at the first prenatal visit), for sexually active gay and bisexual men, and for people with HIV. The shame around syphilis still keeps many people from testing, which is one reason at-home testing has a useful role in widening access.

Trichomoniasis: the parasite often left off panels

Trichomoniasis is caused not by a virus or bacterium but by a single-celled parasite, Trichomonas vaginalis. The CDC estimates around 2.6 million current infections in the U.S. About 70% of people with trich have no symptoms whatsoever, which is one reason it spreads quietly even though it is curable.

Trich passes through contact between vaginal and penile tissue, vaginal-to-vaginal contact, or shared sex toys that haven't been cleaned between uses. When symptoms occur, they include itching, burning, frothy or strong-smelling discharge, painful urination, or discomfort during sex. In pregnancy, untreated trich raises the risk of premature birth and low birth weight, so it is worth specifically asking about during prenatal screening.

A single oral dose of metronidazole or tinidazole clears the infection. Reinfection is common unless partners are treated simultaneously, since the parasite can hide in the male urethra and pass back even when symptoms have resolved. Trich is often left off routine STI panels in primary care, which is one reason at-home testing matters: you can specifically ask for it.

Honest scope note: our at-home trichomoniasis kit is validated for vaginal self-swab only. Male partners who need trich testing should see a clinic, where a urethral swab can be processed.

Herpes (HSV-1 and HSV-2): a contact virus, not a fluid one

Herpes simplex virus comes in two related types. HSV-1 traditionally causes oral cold sores; HSV-2 traditionally causes genital outbreaks. The line has blurred over the past two decades because of oral sex; HSV-1 is now a leading cause of new genital herpes infections in many countries.

Herpes spreads through skin-to-skin contact during kissing, oral, vaginal, or anal sex, including when no sores are visible. The virus sheds asymptomatically, which is why partners can transmit it without ever having an outbreak themselves. When outbreaks occur, they may be painful clusters of small blisters that ulcerate and crust over, or so mild that people mistake them for a yeast infection, ingrown hair, or razor irritation.

There is no cure. Antivirals like valacyclovir, acyclovir, and famciclovir reduce outbreak frequency and viral shedding, lowering the chance of transmission. Daily suppressive therapy is an option for people with frequent outbreaks or those who want to reduce transmission risk to a partner. Condoms reduce transmission risk substantially but do not eliminate it, since the virus can shed from skin not covered by a barrier.

A herpes diagnosis is far more common than stigma suggests, and for most people outbreaks become less frequent over time with antiviral treatment. The virus does remain in the body permanently, and neonatal risk during pregnancy requires specialist guidance, which is why anyone who is pregnant or planning to be should mention a prior herpes diagnosis early in prenatal care. The NHS genital herpes page is a useful starting point on outbreak management and pregnancy considerations.

HIV: the most consequential bloodborne STI, transformed by modern care

HIV (human immunodeficiency virus) transmits through blood, semen, pre-seminal fluid, vaginal fluid, rectal fluid, and breast milk. It is not transmitted through saliva, sweat, tears, casual touching, sharing food, or insect bites (CDC HIV).

The most common routes of new HIV diagnoses in the U.S. are unprotected anal or vaginal sex and the sharing of needles for injection drug use. Mother-to-child transmission during pregnancy, birth, or breastfeeding is largely preventable when the parent is on effective antiretroviral therapy.

Modern HIV care has changed the disease's trajectory completely. Three tools matter:

• PrEP (pre-exposure prophylaxis): daily oral pills or every-two-month injections taken before exposure. Reduces sexual HIV acquisition by roughly 99% with consistent use.
• PEP (post-exposure prophylaxis): a 28-day antiretroviral course started within 72 hours of a possible exposure. Prevents infection if started in time.
• ART (antiretroviral therapy): treatment that suppresses the virus to undetectable levels. People who achieve and maintain an undetectable viral load do not transmit HIV sexually. This is summarized as Undetectable equals Untransmittable, or U=U.

Window periods matter for testing. Antigen/antibody combination tests (the most common laboratory option) detect HIV reliably from about 18 to 45 days after exposure. Rapid antibody tests typically reach reliability around 23 to 90 days. A negative result inside the window doesn't rule out infection, so retesting after the window closes confirms a true negative. If a recent exposure is concerning, talking with a clinician about PEP within 72 hours is the right move; that conversation is time-sensitive.

Hepatitis B: the vaccine-preventable STI most adults skip

Hepatitis B virus (HBV) infects the liver. It spreads through blood, semen, and other body fluids. Sexual transmission is one of the major routes alongside vertical transmission at birth and shared injection equipment. The virus is roughly 50 to 100 times more infectious per exposure than HIV and can survive on surfaces for up to a week (CDC Hepatitis B).

In adults, most acute HBV infections clear on their own within six months. About 5% of newly infected adults develop chronic infection, which can quietly damage the liver over years and lead to cirrhosis or liver cancer. The risk of chronic infection is much higher in infants, which is why birth-dose HBV vaccination has been routine in the U.S. since 1991.

The hepatitis B vaccine is highly effective. The CDC's Advisory Committee on Immunization Practices now recommends universal hepatitis B vaccination for all adults aged 19 to 59, and for adults 60 and older with risk factors. Adults who weren't vaccinated as children can complete the series at any time, and serology testing can confirm immunity if records are unclear.

Symptoms, when they appear, include fatigue, nausea, abdominal pain, jaundice (yellowing of skin or eyes), and dark urine. Many people have no symptoms and only learn they're infected through routine bloodwork or a positive pre-employment screening.

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Mixed swab and fingerstick blood panel covering eight common STIs in a single kit: chlamydia, gonorrhea, syphilis, HIV, hepatitis B, hepatitis C, herpes type 1 (HSV-1), and herpes type 2 (HSV-2). Useful for periodic screening when you want broad coverage in one home session. Confirm any positive with lab testing.

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Asymptomatic spread: the quiet engine of STI transmission

The single biggest reason STIs persist is that most people who have one don't know it. The CDC estimates roughly 90% of people with genital herpes don't realize they have it. About 70% of women with chlamydia have no symptoms. Most HPV infections cause nothing visible. Trichomoniasis is silent in 70% of cases. Even early syphilis often goes unnoticed when the chancre is hidden inside the body.

This stealth factor explains why STIs persist even within monogamous relationships. A partner who feels healthy can still transmit, because feeling fine is not the same as being uninfected. Waiting for symptoms before testing is unreliable as a screening strategy.

What doesn't transmit STIs (despite popular belief)

You can't catch most STIs from:

• Toilet seats
• Sharing cups, utensils, or food
• Hugging, casual cheek kissing, or holding hands
• Pools, hot tubs, gym equipment, or saunas
• Mosquito bites or other insect contact
• Sneezing or coughing in shared spaces

Most STI pathogens are fragile outside the human body. Gonorrhea bacteria die quickly on dry surfaces. HIV is inactivated rapidly outside the bloodstream and can't survive a few minutes in air. Chlamydia can't survive on a toilet seat. Direct contact with infected tissue or fluids is the dominant route precisely because the pathogens don't survive long elsewhere.

A few infections have edge cases worth knowing about:

• Hepatitis B can persist on surfaces for up to a week and is occasionally transmitted through shared razors, toothbrushes, or unsterilized tattoo or piercing equipment.
• Trichomonas can theoretically survive briefly on damp surfaces like shared sex toys if not cleaned between uses.
• HIV and hepatitis B and C can transmit through shared injection drug equipment.

These edge cases are real but uncommon. The vast majority of new infections come from sexual contact. The bigger risk than surfaces is making assumptions: assuming a partner is uninfected because they look healthy, because they say they're tested, or because the relationship feels trustworthy.

Skin-to-skin spread: why condoms aren't a complete shield

Condoms and dental dams are highly effective for STIs that spread through fluids: HIV, chlamydia, gonorrhea, hepatitis B. Where they fall short is the skin-to-skin infections, namely HPV, herpes, and syphilis. Lesions, warts, or shedding skin in the pubic area, scrotum, vulva, perineum, or upper thighs can sit outside what a condom covers.

This is not an argument against condoms. They still substantially reduce risk for everything, and they're the single most accessible prevention tool. It's an argument for layered prevention: HPV and hepatitis B vaccination, routine testing on a schedule that fits your life, honest partner conversations about status and last test date, and PrEP for HIV where appropriate.

For ongoing partners, mutual testing on a regular cadence is the practical version of layered prevention. For new partners or one-time encounters, knowing that protection is excellent but imperfect changes how you think about post-encounter testing windows rather than whether to use protection at all.

More than 1 million curable sexually transmitted infections are acquired every day worldwide, the majority of which are asymptomatic.

Oral sex carries real risk for several STIs

Oral sex is often treated as the safer option. It is lower-risk for some infections and equally risky for others. Herpes, gonorrhea, syphilis, chlamydia, and HPV all transmit through oral-genital contact. HIV transmission via oral sex is rare but possible, particularly when bleeding gums or oral ulcers are present.

Pharyngeal gonorrhea (throat infection) is increasingly common, often asymptomatic, and showing rising antibiotic resistance. Oral syphilis chancres can develop on the lips, tongue, or back of the throat and tend to be missed because they don't hurt. Oral HPV infections are linked to a rising incidence of oropharyngeal cancers, particularly in men.

Dental dams or condoms used during oral sex reduce risk but are uncommon in practice. The reasons are partly cultural and partly because oral has long been framed as the cleaner option.

Putting this into practice

The eight infections covered here behave very differently. Some are curable with a pill or an injection. Some are lifelong but managed with daily medication. Some surface dramatically; most slip past unnoticed. The shared lesson across all of them is that testing is the only reliable way to know your status, since symptoms are not a screening tool. If you've had a new partner in the past three months, if a condom slipped or wasn't used, or if you simply haven't tested in over a year, the next reasonable step is a panel that fits the exposure: a swab-based panel for chlamydia and gonorrhea after a recent contact, and a blood-based panel for HIV and syphilis once those longer windows close.

1. U.S. Centers for Disease Control and Prevention. About sexually transmitted infections, including chlamydia, gonorrhea, and syphilis surveillance and screening recommendations.
2. U.S. Centers for Disease Control and Prevention. About HPV, vaccine recommendations, and transmission information.
3. U.S. Centers for Disease Control and Prevention. HIV topic hub covering transmission routes, prevention with PrEP and PEP, treatment with ART, and U=U.
4. U.S. Centers for Disease Control and Prevention. Hepatitis B virus transmission, vaccine recommendations, and chronic infection risk.
5. World Health Organization. Sexually transmitted infections fact sheet covering daily acquisition rate among adults aged 15 to 49 worldwide and global treatment guidance.
6. NHS. Genital herpes condition page covering outbreak management, antiviral treatment, and pregnancy considerations.

Maya Chen

Maya writes plain-English explainers on STI screening, prevention, and at-home testing. Background in epidemiology research at a state public-health department; articles synthesize CDC and peer-reviewed guidance, not personal clinical advice.