The most common sexually transmitted infections fall into three categories: bacterial (chlamydia, gonorrhea, syphilis), viral (HPV, herpes, HIV, hepatitis B), and parasitic (trichomoniasis). Eight specific infections account for the vast majority of STI cases worldwide, with an estimated 374 million new infections from just four of them each year. Some are curable with a single dose of antibiotics. Others stay in the body permanently but can be managed or prevented with vaccines.
Bacterial STIs
Bacterial STIs are the most treatable. All three major ones can typically be cured with antibiotics, though catching them early matters.
Chlamydia
Chlamydia is the single most reported STI in the United States, with over 1.5 million cases documented in 2024 alone. Globally, there are roughly 129 million new infections per year. The tricky part is that an estimated 77% of chlamydia cases never produce noticeable symptoms, which means many people carry and spread it without knowing. When symptoms do appear, they typically include unusual discharge, burning during urination, or pain during sex.
Left untreated, chlamydia can cause serious damage. About 10 to 15% of women with untreated chlamydia develop pelvic inflammatory disease, which can permanently scar the fallopian tubes and lead to infertility. Chlamydia can also silently infect the upper reproductive tract without causing any obvious symptoms, making routine screening especially important for sexually active people under 25.
Gonorrhea
Gonorrhea caused over 543,000 reported cases in the U.S. in 2024 and roughly 82 million new infections globally per year. Like chlamydia, it often produces no symptoms, particularly in women. An estimated 45% of gonorrhea cases are never symptomatic. When symptoms do show up, they include thick discharge from the genitals, painful urination, and in some cases sore throat or rectal discomfort depending on how the infection was transmitted.
Gonorrhea has become harder to treat over time. The bacteria have developed resistance to nearly every antibiotic once used against them, leaving only one recommended first-line treatment. Untreated gonorrhea, like chlamydia, can lead to pelvic inflammatory disease and infertility.
Syphilis
Syphilis is less common than chlamydia or gonorrhea but has been rising sharply. In 2024, the U.S. recorded over 190,000 total syphilis cases across all stages, including nearly 4,000 cases of congenital syphilis (passed from mother to baby during pregnancy). Globally, about 8 million adults were living with syphilis in 2022.
Syphilis progresses in stages. The first sign is usually a painless sore at the site of infection, which heals on its own after a few weeks. This can trick people into thinking the infection has gone away. Weeks to months later, a second stage may bring rashes, fever, and swollen lymph nodes. If still untreated, syphilis eventually goes quiet for years before potentially causing severe damage to the heart, brain, and other organs. It’s fully curable with antibiotics when caught early.
Viral STIs
Viral STIs cannot be cured, but two of the four major ones are preventable with vaccines, and all can be managed with treatment.
HPV (Human Papillomavirus)
HPV is the most widespread STI in the world. An estimated 300 million women have an active HPV infection at any given time, and most sexually active people will encounter it at some point. The majority of HPV infections clear on their own within a year or two without causing problems. Certain high-risk strains, however, can cause cervical cancer, anal cancer, and cancers of the throat, vulva, and penis. Other strains cause genital warts, which are uncomfortable but not dangerous.
The HPV vaccine is routinely recommended for boys and girls starting at age 11 or 12 and is available through age 26 for anyone not previously vaccinated. Adults aged 27 to 45 may also benefit depending on their risk factors. There is no screening test for HPV in men. In women, Pap smears can detect HPV-related cell changes on the cervix.
Genital Herpes (HSV)
Over 520 million people aged 15 to 49 worldwide are living with genital herpes, caused primarily by herpes simplex virus type 2 (HSV-2). That works out to about 13% of the global population in that age range. Genital herpes causes recurring outbreaks of painful blisters or sores around the genitals or rectum. The first outbreak is usually the worst, and subsequent ones tend to become less frequent over time. Many people with herpes have mild or no symptoms and can unknowingly pass the virus to partners.
There is no cure for herpes, but antiviral medications can shorten outbreaks, reduce their frequency, and lower the risk of transmission.
HIV
HIV attacks the immune system and, without treatment, progresses to AIDS. Early symptoms can resemble the flu: fever, fatigue, swollen glands, and sore throat, typically appearing two to four weeks after exposure. After that initial phase, the virus can remain silent for years while gradually weakening the immune system.
Modern treatment has transformed HIV from a fatal diagnosis into a manageable chronic condition. People on effective treatment can reach an undetectable viral load, meaning they cannot transmit the virus to sexual partners. Preventive medication (PrEP) is also available for people at higher risk of exposure.
Hepatitis B
Hepatitis B infects the liver and can become a lifelong chronic condition. An estimated 254 million people worldwide were living with chronic hepatitis B in 2022, and the virus caused approximately 1.1 million deaths that year, mostly from liver cirrhosis and liver cancer. Many people with hepatitis B have no symptoms for years, which is part of why it spreads so widely.
A highly effective vaccine exists and is part of routine childhood immunization in most countries. It’s also recommended for sexually active adults who haven’t been vaccinated, especially those with multiple partners.
Parasitic STIs
Trichomoniasis
Trichomoniasis is actually the most common curable STI in the world, with an estimated 156 million new infections annually. It’s caused by a tiny parasite rather than a bacterium or virus. In women, symptoms include thin or frothy vaginal discharge that may smell foul, genital itching or soreness, and pain during urination or sex. Men rarely have symptoms, though some experience irritation inside the penis or burning after urination.
Treatment is straightforward: a course of oral antibiotics clears the infection. All sexual partners need to be treated at the same time to prevent reinfection.
Why Many STIs Go Unnoticed
One of the most important things to understand about STIs is that the majority of them can be completely silent. Roughly 77% of chlamydia infections and 45% of gonorrhea infections never produce symptoms. Trichomoniasis, herpes, HPV, and hepatitis B can all be carried and transmitted without the infected person ever feeling sick. This is why STIs spread so efficiently and why testing based on exposure risk, not just symptoms, is the standard approach.
How Long to Wait Before Testing
Testing too soon after exposure can produce a false negative because the infection hasn’t had time to become detectable. The waiting periods vary by infection:
- Chlamydia and gonorrhea: 1 week catches most cases, 2 weeks catches nearly all
- Trichomoniasis: 1 week catches most, 1 month catches nearly all
- Syphilis: 1 month catches most, 3 months catches nearly all
- HIV (blood test): 2 weeks catches most, 6 weeks catches nearly all
- Herpes (blood test): 1 month catches most, 4 months catches nearly all
- Hepatitis B: 3 to 6 weeks
- Hepatitis C: 2 months catches most, 6 months catches nearly all
If you’ve had a specific exposure you’re concerned about, testing at the earlier window and retesting at the later one gives you the most reliable answer.
How STIs Are Prevented
Condoms reduce the risk of HIV transmission by 70 to 80% in heterosexual couples and by 70 to 91% in male same-sex couples when used consistently. They also lower the risk of other STIs, though infections spread through skin contact (like herpes and HPV) can still be transmitted through areas a condom doesn’t cover.
Vaccines are the most effective prevention tool for two major viral STIs. The HPV vaccine prevents the strains responsible for most HPV-related cancers and genital warts. The hepatitis B vaccine provides long-lasting protection against chronic liver infection. Both are widely available and recommended as part of routine healthcare.