A sexually transmitted infection, or STI, is a virus, bacterium, fungus, or parasite that spreads through sexual contact. The term covers more than 20 different infections, ranging from extremely common and easily curable to lifelong conditions that require ongoing management. Over 2.2 million cases of just three STIs (chlamydia, gonorrhea, and syphilis) were reported in the United States in 2024 alone.
STI vs. STD: What’s the Difference?
You’ll see both terms used interchangeably, but they mean slightly different things. An STI refers to the infection itself: a pathogen that has entered your body. An STD, or sexually transmitted disease, means that infection has progressed enough to cause noticeable symptoms or damage. In practice, many people carry an STI without ever developing what a doctor would call a “disease.” The medical community has largely shifted toward using “STI” because it’s more accurate and because it reinforces an important point: you can be infected and pass an infection to someone else without ever feeling sick.
How STIs Spread
STIs primarily spread through vaginal, anal, and oral sex. Some travel through bodily fluids like semen, vaginal secretions, or blood. Others, like herpes and HPV, spread through skin-to-skin contact with an infected area, which means a condom reduces risk but doesn’t eliminate it entirely.
Oral sex carries real transmission risk for several infections, including syphilis, gonorrhea, herpes, and HPV. You can get an STI in the mouth or throat from giving oral sex to an infected partner, and you can get one on the genitals from receiving it. Oral-anal contact can also transmit hepatitis A and B. HIV risk from oral sex, by contrast, is very low.
Factors that increase transmission risk during any type of sex include open sores on the mouth or genitals, bleeding gums or poor oral health, and contact with pre-ejaculate or ejaculate. Critically, STIs can spread even when neither partner has any visible signs or symptoms.
The Most Common Types
STIs fall into three categories based on what causes them, and the category determines whether the infection is curable.
Bacterial STIs
Chlamydia is the most frequently reported STI in the U.S., with over 1.5 million cases in 2024. Gonorrhea accounted for another 543,000 cases, and syphilis about 190,000. All three are curable with antibiotics, though gonorrhea is becoming harder to treat as the bacteria develop drug resistance. Trichomoniasis, caused by a parasite, is also curable and is actually the most common curable STI worldwide, though it’s less consistently reported in surveillance data.
Reinfection is common with all of these. Clearing an infection once doesn’t protect you from getting it again.
Viral STIs
HPV (human papillomavirus), genital herpes, HIV, and hepatitis B and C are caused by viruses. There is no cure for viral STIs, but antiviral medications can manage symptoms, reduce the frequency of outbreaks, and significantly lower the risk of passing the infection to others. HPV often clears on its own within a couple of years, but certain strains can lead to cancer if they persist. HIV, once a death sentence, is now a manageable chronic condition with daily medication.
Why Many People Don’t Know They’re Infected
One of the most important things to understand about STIs is that most of them produce no symptoms at all, at least initially. Roughly 77% of chlamydia cases and 45% of gonorrhea cases never cause noticeable symptoms. Herpes can also remain dormant for months or years. This is why STIs spread so effectively: people who feel perfectly healthy pass infections to partners without realizing it.
When symptoms do appear, they vary widely depending on the infection. They can include unusual discharge, burning during urination, sores or bumps on the genitals, itching, or pain during sex. But the absence of symptoms means nothing on its own. The only way to know your status is testing.
Testing and Window Periods
Getting tested too soon after exposure can produce a false negative because the infection hasn’t had time to become detectable. Each STI has a different window period:
- Chlamydia and gonorrhea: Generally detectable within one to two weeks.
- Syphilis: A blood test catches most cases at one month, and nearly all by three months.
- HIV: A blood test using the antigen/antibody method catches most infections at two weeks, with near-complete accuracy by six weeks. An oral swab takes longer, catching most at one month and nearly all by three months.
- Hepatitis B: Detectable at three to six weeks.
- Hepatitis C: A blood test catches most cases at two months, but full confidence requires waiting up to six months.
If you’ve had a specific exposure you’re concerned about, the timing of your test matters. Testing at the right interval gives you a much more reliable result than testing the next day.
What Happens Without Treatment
Untreated STIs can cause serious long-term damage, and the consequences are often worse for women. Chlamydia and gonorrhea can lead to pelvic inflammatory disease, a painful infection of the reproductive organs that can cause chronic pelvic pain, ectopic pregnancy (where a fertilized egg implants outside the uterus), and infertility.
Syphilis progresses through stages and can eventually damage the brain, heart, and other organs. In pregnant women, it’s particularly dangerous: untreated syphilis accounts for an estimated 29% of perinatal deaths and 26% of stillbirths. Nearly 4,000 cases of congenital syphilis (passed from mother to baby) were reported in the U.S. in 2024.
Persistent HPV infection is the primary cause of cervical cancer. And herpes, while not life-threatening for most adults, substantially increases vulnerability to HIV. Research estimates that up to 50% of new HIV infections in people with herpes can be attributed to the way herpes creates entry points for the virus.
Prevention and Vaccines
Condoms, used consistently, are the most effective barrier against most STIs, though they’re less protective against infections spread by skin contact (herpes, HPV, syphilis sores outside the covered area). Reducing the number of sexual partners and having open conversations about testing also lower risk.
Two STIs are preventable with vaccines. The HPV vaccine is recommended at ages 11 to 12, though it can be given as early as 9. At that age, only two doses are needed, spaced 6 to 12 months apart. People who start the series at 15 or older need three doses. The vaccine is available up to age 26, and some adults between 27 and 45 may benefit from it after discussing their situation with a doctor. The hepatitis B vaccine is part of the standard childhood immunization schedule and is also available to unvaccinated adults.
For HIV specifically, a preventive medication called PrEP reduces the risk of infection by about 99% when taken as prescribed. It’s available as a daily pill or a long-acting injection given every two months.