Sexually transmitted infections (STIs) are infections caused by viruses, bacteria, fungi, or parasites that pass from one person to another through sexual contact. They are extremely common: the World Health Organization estimated 374 million new infections in 2020 from just four curable STIs alone. Many STIs produce no symptoms at all, which means people can carry and spread them without knowing.
STI vs. STD: Why the Name Changed
You’ll still see “STD” (sexually transmitted disease) used in everyday conversation, but most health organizations now prefer “STI.” The distinction matters. An infection means a virus or bacterium has entered your body. A disease means that infection has progressed far enough to cause noticeable symptoms or health problems. Since the goal is to catch and treat these infections before they cause disease, the shift to “STI” reflects a focus on early detection. Many people with STIs never develop symptoms, so calling every case a “disease” was misleading.
How STIs Spread
STIs spread most commonly during vaginal, oral, or anal sex through contact with infected bodily fluids like semen, vaginal secretions, or blood. But not all STIs require fluid exchange. Herpes and HPV spread through skin-to-skin contact, meaning any sexual activity involving the genitals, mouth, or anus can transmit them, even without penetration.
Some STIs also spread through non-sexual routes. A pregnant person with syphilis can pass the infection to their baby during pregnancy or birth. In 2022, an estimated 1.1 million pregnant women had syphilis, resulting in over 390,000 adverse birth outcomes worldwide. Hepatitis B and HIV can also pass from parent to child during delivery or breastfeeding.
Bacterial STIs
Bacterial STIs are caused by bacteria and are curable with antibiotics when caught early. The three most significant ones are chlamydia, gonorrhea, and syphilis.
Chlamydia is the most common curable STI globally, with an estimated 129 million new cases in 2020. It often produces no symptoms, particularly in women. When symptoms do appear, they typically include unusual discharge, burning during urination, or pelvic pain. Left untreated, chlamydia can cause pelvic inflammatory disease, which can lead to chronic pain and infertility.
Gonorrhea accounted for roughly 82 million new infections in 2020. Symptoms overlap with chlamydia (discharge, painful urination, pelvic discomfort) and are more likely to appear in men than women. Gonorrhea has become a growing concern because some strains are developing resistance to standard antibiotics, making treatment more complicated.
Syphilis progresses through distinct stages. It starts as a painless sore (often unnoticed), then moves to a rash and flu-like symptoms, and can eventually damage the brain, heart, and other organs if left untreated for years. About 8 million adults between 15 and 49 were infected with syphilis in 2022. It is fully curable in its early stages.
Viral STIs
Viral STIs can be managed and treated, but most cannot be fully cured. The virus stays in your body, though treatment can reduce symptoms and lower the risk of spreading it to others.
HPV (human papillomavirus) is the most common STI overall. Most people who are sexually active will contract at least one strain of HPV at some point. Many strains clear on their own within a year or two, but certain high-risk strains can cause cervical, throat, and anal cancers. Screening is important: guidelines recommend women get a Pap test every three years starting at age 21, and after age 30, a combined HPV and Pap test every five years.
Herpes (HSV) causes recurring outbreaks of painful blisters or sores around the mouth or genitals. Outbreaks tend to become less frequent over time. Daily antiviral therapy can significantly reduce the number of recurrences and lower the chance of passing the virus to a partner.
HIV attacks the immune system and, without treatment, progresses to AIDS. Modern antiviral treatment has transformed HIV from a fatal diagnosis into a manageable chronic condition. When someone takes their medication consistently, the virus can become undetectable in the blood, at which point they effectively cannot spread it to sexual partners. Starting treatment early makes it more effective.
Hepatitis B and C are liver infections that can become chronic. Hepatitis B has a vaccine, while hepatitis C now has highly effective antiviral treatments that cure the infection in most cases.
Parasitic STIs
Trichomoniasis is caused by a microscopic parasite and was responsible for an estimated 156 million new infections in 2020, making it the most common curable STI by sheer numbers. Symptoms include itching, burning, and unusual discharge, though many people (especially men) have no symptoms. A single course of prescription antibiotics typically clears it.
Pubic lice (sometimes called “crabs”) are tiny insects that live in coarse body hair. They cause intense itching and are treatable with over-the-counter lotions containing permethrin. A second treatment 9 to 10 days later may be needed to kill any remaining lice.
Why Many STIs Go Unnoticed
One of the most important things to understand about STIs is that many produce no obvious symptoms, sometimes for months or years. Chlamydia, gonorrhea, HPV, and even HIV can all be present in your body without any outward signs. This is why STIs spread so easily: people don’t get tested because they feel fine, and they unknowingly pass the infection to partners. Regular testing is the only reliable way to know your status, especially if you have new or multiple sexual partners.
Testing Window Periods
Getting tested too soon after exposure can produce a false negative because your body hasn’t generated enough of a response to detect. Each STI has a different “window period” before a test becomes reliable:
- HIV (blood test): 2 weeks catches most cases; 6 weeks catches nearly all
- HIV (oral swab): 1 month catches most; 3 months catches nearly all
- Syphilis: 1 month catches most; 3 months catches nearly all
- Hepatitis B: 3 to 6 weeks
- Hepatitis C: 2 months catches most; 6 months catches nearly all
Chlamydia and gonorrhea can typically be detected within 1 to 2 weeks of exposure through urine or swab tests. If you think you were exposed, timing your test correctly makes a significant difference in accuracy.
Prevention
Condoms are highly effective at preventing STIs that spread through bodily fluids, like HIV, gonorrhea, and chlamydia. They are less protective against infections that spread through skin-to-skin contact, like herpes and syphilis, because the infected area may not be covered. Still, consistent condom use substantially reduces overall risk.
The HPV vaccine is one of the most effective prevention tools available. It is routinely recommended at age 11 or 12, can be given as early as 9, and is recommended for everyone through age 26. Adults between 27 and 45 can also receive it after discussing the benefits with a provider. Children who get their first dose before age 15 need only two shots, while those starting at 15 or older need three.
PrEP (pre-exposure prophylaxis) is a daily or on-demand medication that reduces the risk of getting HIV by over 99% when taken consistently. It is available for anyone at higher risk of HIV exposure.
What Happens if STIs Go Untreated
Untreated bacterial STIs can cause serious, sometimes irreversible damage. Chlamydia and gonorrhea can lead to pelvic inflammatory disease in women, which scars the reproductive organs and can cause infertility or ectopic pregnancies. In men, untreated infections can lead to painful inflammation of the reproductive tract. Syphilis can progress to damage the brain, nerves, eyes, and heart.
Untreated viral STIs carry their own risks. Certain HPV strains are responsible for the majority of cervical cancers. Chronic hepatitis B or C can lead to liver cirrhosis and liver cancer. HIV, without treatment, destroys immune function over several years. In all of these cases, early detection and treatment dramatically reduce the chance of long-term harm.