A sexually transmitted infection (STI) is an infection caused by a virus, bacterium, fungus, or parasite that passes from one person to another through sexual contact. Globally, over 1 million new cases of curable STIs occur every day among adults aged 15 to 49. The term “STI” has largely replaced “STD” (sexually transmitted disease) in medical and public health settings because most of these infections can be caught and treated before they ever cause symptoms or develop into a disease.
STI vs. STD: Why the Name Changed
The shift from “STD” to “STI” reflects an important distinction. An infection means a pathogen has entered your body, but it may not be causing any noticeable problems yet. A disease means that infection has progressed to the point where it produces symptoms. Since many sexually transmitted infections never cause obvious symptoms at all, calling them “diseases” was misleading. The CDC and most health organizations now prefer “STI” to emphasize that early detection and treatment can stop an infection before it becomes something more serious.
The Most Common Types
STIs fall into a few categories based on what’s causing them, and the type of pathogen determines whether the infection can be cured or only managed.
Bacterial STIs
These are caused by bacteria and can be fully cured with antibiotics. The most common are chlamydia, gonorrhea, and syphilis. In 2020, the World Health Organization estimated 374 million new infections of these three plus trichomoniasis among adults worldwide. Syphilis alone accounted for 8 million new infections in 2022.
Viral STIs
Viral infections include HIV, herpes (HSV), human papillomavirus (HPV), and hepatitis B. These cannot be cured with antibiotics. Some, like herpes and HIV, stay in your body permanently but can be managed with medication to reduce symptoms and prevent transmission. HPV infections often clear on their own within a couple of years, but certain strains can lead to genital warts or cancer if they persist.
Parasitic STIs
Trichomoniasis is the most common parasitic STI. It’s caused by a single-celled organism and is curable with prescription medication. Like many STIs, it often produces no symptoms, especially in men.
How STIs Spread
STIs can be transmitted through vaginal, anal, or oral sex. The pathogens enter the body through tiny, often invisible breaks in the moist lining (mucous membranes) of the genitals, mouth, or rectum. You don’t need to have a visible cut or sore for transmission to happen.
Some STIs spread primarily through bodily fluids like semen, vaginal secretions, or blood. HIV is the clearest example, and it can also spread through shared needles or from parent to child during birth or breastfeeding. Others, like herpes, syphilis, and HPV, spread through direct skin-to-skin contact with an infected area, which means condoms reduce but don’t fully eliminate the risk for those infections.
Why Most People Don’t Know They Have One
One of the most important things to understand about STIs is that the majority of infected people feel completely fine. Research shows that roughly 61% of chlamydia infections, 53% of gonorrhea infections, and 57% of trichomoniasis infections produce no symptoms at all in women. The numbers are similarly high in men for certain infections, particularly chlamydia.
This is exactly why STIs spread so effectively. Someone with no symptoms has no reason to suspect they’re infected, so they don’t get tested and may unknowingly pass the infection to partners. It’s also why routine screening matters even when you feel healthy.
What Happens if an STI Goes Untreated
Without treatment, even a “silent” infection can cause real damage over time. In women, untreated chlamydia or gonorrhea can spread to the uterus and fallopian tubes, causing pelvic inflammatory disease (PID). PID creates scar tissue that can block the fallopian tubes, leading to chronic pelvic pain, ectopic pregnancy (where a fertilized egg implants outside the uterus), or infertility. About 1 in 8 women with a history of PID have difficulty getting pregnant.
Untreated syphilis progresses through stages over years and can eventually damage the brain, heart, and other organs. Untreated HPV infections with high-risk strains can lead to cervical, throat, or anal cancers. Untreated HIV gradually destroys the immune system, leading to AIDS. In 2022, untreated syphilis during pregnancy resulted in an estimated 700,000 cases of congenital syphilis in newborns worldwide.
How STIs Are Detected
Testing varies by infection but is generally simple and painless. For chlamydia and gonorrhea, the standard is a urine sample or a swab of the affected area (vagina, rectum, or throat). For HIV, syphilis, and hepatitis B, a blood test is used. Some clinics also offer rapid oral swab tests for HIV.
Modern tests that amplify the genetic material of the pathogen are extremely accurate. For chlamydia, these tests detect as few as one organism per sample, with sensitivity of 96% or higher and specificity above 99%. Older testing methods miss 10 to 30% more infections by comparison.
Testing Window Periods
Getting tested too soon after exposure can produce a false negative because the infection hasn’t built up enough to be detectable. Each STI has its own window period:
- Chlamydia: 1 week catches most infections, 2 weeks 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
- HIV (oral swab): 1 month catches most, 3 months catches nearly all
If you’re concerned about a specific exposure, timing your test based on these windows gives you the most reliable result.
Treatment Options
Bacterial STIs like chlamydia, gonorrhea, and syphilis are cured with antibiotics. Chlamydia typically requires a week-long course of oral antibiotics. Gonorrhea is treated with a single injection. These are straightforward treatments, and the infection clears completely once you finish.
Viral STIs can’t be cured, but they can be managed effectively. Antiviral medications for herpes reduce the frequency and severity of outbreaks and lower the chance of passing it to a partner. HIV treatment (antiretroviral therapy) can reduce the virus to undetectable levels, which means it’s untransmittable to sexual partners. Hepatitis B is managed with monitoring and, in some cases, antiviral medication.
Trichomoniasis is cured with a single dose of prescription anti-parasitic medication.
Prevention: What Actually Works
Condoms, when used consistently and correctly, significantly reduce the risk of most STIs. They’re most effective against fluid-borne infections like HIV, gonorrhea, and chlamydia. For skin-contact infections like herpes and HPV, condoms lower risk but don’t cover all potentially infectious skin.
Vaccination is one of the most effective prevention tools available. The HPV vaccine is recommended for everyone starting at age 11 or 12 and is available through age 26 for those who haven’t been vaccinated. It prevents the strains of HPV most likely to cause cancer and genital warts. Hepatitis B vaccination is recommended for all sexually active people who haven’t been previously vaccinated, and hepatitis A vaccination is recommended for certain higher-risk groups.
Regular screening is prevention too. Because so many STIs are asymptomatic, testing at appropriate intervals catches infections before they cause damage or spread to others. For sexually active people with new or multiple partners, annual screening for chlamydia and gonorrhea is a reasonable baseline, with HIV and syphilis testing based on your individual risk factors.