A sexually transmitted infection, or STI, is a virus, bacterium, fungus, or parasite that spreads from one person to another through sexual contact. Globally, more than one million new cases occur every day among adults aged 15 to 49, totaling roughly 374 million new infections of just the four most common types each year. STIs are extremely widespread, often completely silent, and in most cases either curable or manageable with treatment.
STI vs. STD: Why the Language Changed
You’ll still see the term “sexually transmitted disease,” or STD, but health organizations including the CDC now prefer “sexually transmitted infection.” The distinction matters: an infection means a pathogen has entered the body, while a disease means that infection has progressed enough to cause noticeable symptoms or damage. Since the whole point of screening and treatment is to catch infections before they become diseases, the newer term better reflects that goal. Many people carry an STI for months or years without ever developing what a doctor would call a disease.
How STIs Spread
STIs can pass through vaginal, anal, and oral sex. Oral contact with the genitals or anus can transmit infections in both directions: a genital infection can move to a partner’s mouth or throat, and a mouth or throat infection can spread to the genitals. Oral-anal contact specifically can also transmit hepatitis A and B, along with intestinal parasites.
Skin-to-skin contact plays a role too. Infections like herpes and syphilis spread through direct contact with sores or infected skin, which means condoms reduce but don’t eliminate the risk for these particular STIs. Critically, transmission can happen even when neither partner has any visible signs or symptoms.
Common Bacterial STIs
Bacterial STIs are curable with antibiotics, though reinfection is common if partners aren’t treated at the same time.
Chlamydia is one of the most frequently diagnosed STIs, and most people who have it experience no symptoms at all. When symptoms do appear, painful urination is the most typical sign. Left untreated, about 10 to 15 percent of women with chlamydia develop pelvic inflammatory disease (PID), which can permanently damage the fallopian tubes and lead to infertility. Chlamydia can also cause this damage silently, without any obvious warning signs.
Gonorrhea can infect the genitals, rectum, and throat. Like chlamydia, it’s a leading preventable cause of PID and infertility. Treatment currently requires two drugs, and resistance to one of them is already developing, which makes prompt treatment especially important.
Syphilis progresses in stages if untreated. The first stage produces painless sores on the genitals, mouth, or rectum. The second stage brings a rash and swollen lymph nodes. If it reaches the third stage, syphilis can damage the brain, heart, and other organs. A simple course of penicillin cures it. In 2022, there were 8 million new syphilis infections worldwide and an estimated 700,000 cases of congenital syphilis, where the infection passes from mother to baby during pregnancy.
Trichomoniasis is caused by a parasite rather than a bacterium but is similarly curable. Symptoms can include discharge and irritation, though many infections go unnoticed.
Common Viral STIs
Viral STIs cannot be cured, but they can be managed effectively with medication.
Human papillomavirus (HPV) is the most widespread STI. Most infections clear on their own, but certain strains can cause genital warts and, over time, lead to cervical, throat, and other cancers. A vaccine prevents the most dangerous strains and is recommended for everyone through age 26, with the option to vaccinate up to age 45 after a conversation with a provider.
Herpes simplex virus (HSV) causes recurring sores on or around the genitals or mouth. Antiviral medications reduce the frequency and severity of outbreaks and lower the chance of passing the virus to a partner, but the virus remains in the body permanently.
HIV attacks the immune system and, without treatment, progresses to AIDS. Modern antiviral therapy can suppress the virus to undetectable levels, allowing people with HIV to live long, healthy lives and effectively eliminating the risk of sexual transmission.
Hepatitis B infects the liver and can become chronic. Antivirals help slow liver damage, and a highly effective vaccine is available for prevention. Routine hepatitis B vaccination is recommended for all adults through age 59.
Why So Many Cases Go Unnoticed
One of the biggest challenges with STIs is that they frequently produce no symptoms. Oral and rectal chlamydia and gonorrhea infections are asymptomatic roughly 91 to 92 percent of the time. Even genital infections often fly under the radar: about 25 percent of genital chlamydia and gonorrhea cases produce no noticeable signs. For syphilis, around 61 percent of cases caught through routine screening are asymptomatic. This is exactly why regular testing matters so much: waiting for symptoms to appear means many infections go undiagnosed and continue spreading.
How Testing Works
STI testing is straightforward and varies depending on the infection. Blood tests are used to diagnose syphilis, HIV, hepatitis B, and sometimes herpes. Urine samples can detect chlamydia, gonorrhea, and trichomoniasis. Swab tests, where a provider collects a sample from the vagina, penis, rectum, or throat, diagnose HPV, chlamydia, gonorrhea, and herpes. In rare cases where syphilis or herpes may have affected the brain or spinal cord, a spinal fluid test is used.
Testing Window Periods
Tests aren’t accurate immediately after exposure. Each infection has a window period, the minimum time it takes for the test to reliably detect it. Getting tested too early can produce a false negative.
- Chlamydia and gonorrhea: 1 week detects most cases; 2 weeks catches nearly all
- Syphilis: 1 month detects most; 3 months catches nearly all
- HIV (blood antigen/antibody 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
- Trichomoniasis: 1 week detects most; 1 month catches nearly all
- Hepatitis B: 3 to 6 weeks
- Hepatitis C: 2 months detects most; 6 months catches nearly all
Long-Term Risks of Untreated Infections
When STIs go untreated, the consequences extend far beyond the initial infection. Chlamydia and gonorrhea can cause pelvic inflammatory disease, which scars the fallopian tubes and surrounding tissue, sometimes leading to infertility or life-threatening ectopic pregnancies. This damage can happen without any symptoms along the way. Untreated syphilis can progress over years to affect the heart, brain, and nervous system. Certain HPV strains, left unchecked by the immune system, can develop into cancer over a decade or more. HIV, without treatment, steadily destroys the immune system.
The common thread is that early detection transforms the outlook. Bacterial infections are curable. Viral infections are manageable. The real danger comes from infections no one knows about.
Prevention Beyond Condoms
Condoms remain one of the most effective tools for reducing STI transmission, but they don’t cover all the skin where infections like herpes and syphilis can live. That makes other prevention strategies important too.
Vaccines offer powerful protection against specific STIs. The HPV vaccine is recommended as a routine series for everyone through age 26. Hepatitis B vaccination is routine for adults through age 59. Hepatitis A vaccination is recommended for anyone at increased risk, including men who have sex with men and people living with HIV.
Regular screening is itself a form of prevention. Because so many infections are asymptomatic, routine testing catches cases that would otherwise spread silently to partners. Treating infections early also prevents the complications that make long-term damage possible. For HIV specifically, pre-exposure prophylaxis (PrEP) is available for people at higher risk and is highly effective at blocking transmission before it happens.