A sexually transmitted infection (STI) is an infection passed from one person to another through sexual contact, including vaginal, oral, and anal sex. STIs are extremely common: the World Health Organization estimated 374 million new infections with just four curable STIs in 2020 alone. Some STIs cause noticeable symptoms, but many do not, which is why routine testing matters even when you feel fine.
How STIs Spread
STIs spread primarily through vaginal, oral, and anal sex. During these activities, bacteria, viruses, or parasites transfer between partners through bodily fluids like semen, vaginal secretions, and blood, or through direct skin-to-skin contact with an infected area. Some STIs, like herpes and HPV, spread through skin contact alone and don’t require the exchange of fluids.
Less commonly, STIs can spread through intimate physical contact like heavy petting, though this is rare. A few STIs, notably hepatitis B and syphilis, can also pass from a pregnant person to their baby during pregnancy or delivery.
Common Types of STIs
STIs fall into three broad categories based on what causes them, and this distinction matters because it determines whether the infection can be cured or only managed.
Bacterial STIs (Curable)
Bacterial infections respond to antibiotics and can be fully eliminated. The most common ones are chlamydia (129 million new cases globally in 2020), gonorrhea (82 million), and syphilis (7.1 million). Once treated, these infections are gone, though you can get reinfected through future exposure.
Viral STIs (Manageable, Not Curable)
Viral STIs can be controlled with medication but stay in your body long-term. Over 520 million people worldwide are living with genital herpes. An estimated 300 million women have an HPV infection, and 254 million people were living with chronic hepatitis B as of 2022. HIV also falls into this category. Antiviral treatments can suppress symptoms and reduce the risk of passing these infections to others, but they don’t eliminate the virus entirely.
Parasitic STIs (Curable)
Trichomoniasis is the most common parasitic STI, with 156 million new infections estimated in 2020. It’s caused by a tiny parasite and is easily treated with a single course of medication. Pubic lice are another parasitic STI, treated with topical solutions.
Why Many STIs Go Unnoticed
One of the most important things to understand about STIs is that the majority of infected people have no symptoms at all. Research estimates that roughly 77% of chlamydia cases and 45% of gonorrhea cases never produce symptoms. This means a person can carry and transmit an infection for months or years without knowing it.
When symptoms do appear, they vary by infection but commonly include unusual discharge from the genitals, burning during urination, sores or blisters on or around the genitals, itching, and pelvic pain. Some viral STIs like HPV produce no symptoms in most people but can still lead to serious health problems years later, including cervical cancer.
What Happens If an STI Goes Untreated
Left untreated, bacterial STIs in particular can cause serious long-term damage. Untreated chlamydia or gonorrhea can lead to pelvic inflammatory disease (PID) in women, a condition where the infection spreads to the uterus and fallopian tubes. PID creates scar tissue that can block the fallopian tubes, cause chronic pelvic pain, and lead to ectopic pregnancies (where a fertilized egg implants outside the uterus, a medical emergency). One in eight women with a history of PID has difficulty getting pregnant. The longer treatment is delayed, the more likely these complications become.
Untreated syphilis progresses through stages over years and can eventually damage the brain, heart, and other organs. Untreated gonorrhea can spread to the bloodstream and joints. Having any untreated STI also increases your vulnerability to HIV, because the inflammation or open sores created by one infection make it easier for HIV to enter the body.
How STI Testing Works
Testing typically involves a blood draw, a urine sample, or a swab from the vagina, throat, or rectum, depending on the infection and the type of sexual contact you’ve had. There’s no single test that checks for every STI at once, so your testing panel depends on your risk factors and sexual history.
Self-testing options are now available for several STIs. There are at-home tests for HIV and syphilis that give results within minutes from a finger prick or cheek swab. Self-collection kits for chlamydia, gonorrhea, and trichomoniasis let you take your own vaginal swab or urine sample and mail it to a lab for results within a few days.
Testing Window Periods
STIs don’t show up on tests immediately after exposure. Each infection has a “window period,” the time it takes for the infection to become detectable. Testing too early can produce a false negative. Here’s how long to wait after a potential exposure:
- Chlamydia and gonorrhea: 1 week detects most cases, 2 weeks catches nearly all
- Trichomoniasis: 1 week detects most, 1 month catches nearly all
- Syphilis: 1 month detects most, 3 months catches nearly all
- HIV (blood test): 2 weeks detects most, 6 weeks catches nearly all
- HIV (oral swab): 1 month detects most, 3 months catches nearly all
- Herpes (blood test): 1 month detects most, 4 months catches nearly all
- Hepatitis B: 3 to 6 weeks
- Hepatitis C: 2 months detects most, 6 months catches nearly all
- HPV (Pap smear): 3 weeks to a few months
Some infections, including genital warts (caused by certain HPV strains) and pubic lice, have no screening test and are diagnosed by visual examination.
Prevention and Vaccines
Condoms significantly reduce the risk of most STIs when used consistently, though they’re less effective against infections that spread through skin contact in areas a condom doesn’t cover, like herpes and HPV.
Vaccines are available for two of the most serious viral STIs. The HPV vaccine is recommended for everyone starting at age 11 or 12 (and can be given as early as age 9), with catch-up vaccination available through age 26. Adults between 27 and 45 who weren’t vaccinated earlier can discuss it with a healthcare provider. The HPV vaccine is one of the most effective tools for preventing HPV-related cancers. 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.
Beyond vaccines and condoms, practical steps that lower risk include reducing your number of sexual partners, getting tested regularly (and asking partners to do the same), and avoiding sexual contact when sores or symptoms are present. For people at higher risk of HIV, pre-exposure prophylaxis (PrEP) is a daily medication that reduces the chance of contracting HIV by more than 99% when taken as directed.
STI vs. STD: Is There a Difference?
You’ll see both “STI” and “STD” used interchangeably, and for practical purposes they refer to the same group of conditions. The shift toward “STI” in medical and public health language reflects the fact that most of these conditions are infections that may never progress to cause disease (the “D” in STD). Using “infection” is also considered less stigmatizing and more medically accurate, since many people with STIs have no symptoms and no disease process occurring. The CDC and most major health organizations now use “STI” as the preferred term.