A sexually transmitted infection (STI) is a virus, bacterium, fungus, or parasite that spreads from person to person through sexual contact. More than 1 million curable STIs are acquired every day worldwide among people aged 15 to 49, and the majority cause no symptoms at all. That combination of high prevalence and silent spread is what makes STIs a significant public health concern and why understanding them matters for anyone who is sexually active.
You may have seen the terms STI and STD used interchangeably. The distinction is straightforward: an STI refers to the presence of the infectious organism in your body, while an STD (sexually transmitted disease) means that infection has progressed enough to cause noticeable symptoms or health problems. Someone can carry an STI for months or years without it ever becoming a “disease” in the traditional sense, which is why the term STI is now preferred in most clinical settings.
How STIs Spread
STIs pass between people through two main routes, depending on the type of organism involved. Some spread through bodily fluids: blood, semen, vaginal secretions, or other fluids exchanged during vaginal, anal, or oral sex. HIV, chlamydia, gonorrhea, and hepatitis B all fall into this category. Others spread through direct skin-to-skin contact with an infected area, even when no fluids are exchanged. Herpes and HPV (human papillomavirus) are the most common examples. This is why condoms and dental dams reduce the risk of most STIs but offer less protection against infections that involve exposed sores or skin areas not covered by the barrier.
A few STIs can also spread through non-sexual routes. Hepatitis B and HIV can pass from mother to baby during childbirth or through shared needles. Hepatitis A can spread through contaminated food or water, though sexual transmission is also common.
Most STIs Cause No Symptoms
This is the single most important thing to understand about sexually transmitted infections: the majority are asymptomatic. You can carry and transmit an infection without ever feeling sick, noticing a sore, or experiencing unusual discharge. When symptoms do appear, they often mimic other conditions, making them easy to dismiss.
That said, when STIs do produce symptoms, certain patterns are common. Bacterial infections like chlamydia and gonorrhea can cause burning during urination, unusual genital discharge, or pain during sex. Syphilis often begins as a painless sore at the site of infection, followed weeks later by a rash. Herpes produces clusters of painful blisters that heal and then recur. Genital warts from HPV appear as small, flesh-colored bumps. HIV may cause flu-like symptoms within a few weeks of infection, then go silent for years.
Because so many infections produce no obvious signs, regular testing is the only reliable way to know your status.
Common Bacterial STIs
Bacterial STIs are curable with the right treatment, which is the good news. The bad news is that they often go undetected long enough to cause complications.
Chlamydia is the most frequently reported STI in the United States, with over 1.5 million cases documented in 2024 alone. It rarely causes symptoms, particularly in women. Left untreated, about 10 to 15 percent of women with chlamydia develop pelvic inflammatory disease (PID), an infection of the uterus and fallopian tubes that can cause permanent damage and lead to infertility.
Gonorrhea accounted for more than 543,000 reported U.S. cases in 2024. It affects similar areas as chlamydia and can also infect the throat and rectum. Gonorrhea has become increasingly difficult to treat because the bacterium has developed resistance to multiple classes of antibiotics over the decades. Current treatment relies on a single injectable antibiotic, and when that fails, backup options are limited.
Syphilis progresses through distinct stages if untreated: a painless sore, then a body rash, then a long latent period, and finally damage to the brain, heart, and other organs. There were over 190,000 total syphilis cases reported in the U.S. in 2024, a figure that includes congenital syphilis passed from mothers to newborns during pregnancy. Syphilis is treatable, but the damage from late-stage infection can be irreversible.
Common Viral STIs
Viral STIs generally cannot be cured, but most can be managed effectively, and some can be prevented with vaccines.
HPV is the most common STI overall. Most people who are sexually active will encounter it at some point. The majority of HPV infections clear on their own within a year or two, but certain high-risk strains can cause cervical, throat, anal, and penile cancers. A vaccine is available and recommended for all children at age 11 or 12, with catch-up vaccination through age 26 for those not previously vaccinated. Vaccination is one of the most effective tools available for preventing HPV-related cancers.
Herpes (HSV) causes recurring outbreaks of painful blisters, though many people with herpes experience mild or no symptoms. There is no cure, but antiviral medications reduce the frequency and severity of outbreaks. Herpes can be transmitted even when no visible sores are present.
HIV attacks the immune system and, without treatment, progresses to AIDS. Modern antiretroviral therapy has transformed HIV from a fatal diagnosis into a manageable chronic condition. People on effective treatment who maintain an undetectable viral load have zero risk of transmitting HIV to sexual partners. That finding has reshaped both treatment and prevention strategies. A daily preventive medication (PrEP) is also available for people who are HIV-negative but at higher risk of exposure.
Hepatitis B can cause chronic liver infection and long-term liver damage. Like HPV, it is preventable through vaccination. The hepatitis B vaccine is recommended for all unvaccinated people who are sexually active with more than one partner or who are being evaluated for any STI.
Testing and Window Periods
STI tests are not one-size-fits-all. Different infections require different types of tests, and each has a “window period,” the time between exposure and when the test can reliably detect the infection. Testing too early can produce a false negative.
- HIV (blood test): Detectable in most people within 2 weeks using antigen/antibody testing. Nearly all cases are caught by 6 weeks.
- HIV (oral swab): Takes longer. Most infections are detectable by 1 month, with nearly all caught by 3 months.
- Syphilis: Blood tests catch most infections within 1 month, and almost all by 3 months.
- Hepatitis B: Detectable by blood test within 3 to 6 weeks.
- Hepatitis C: Most infections show up by 2 months, but full confidence requires waiting 6 months.
- Chlamydia and gonorrhea: Typically detectable within 1 to 2 weeks after exposure using urine samples or swabs.
If you’ve had a recent exposure, your provider may recommend testing at the earliest reliable window and then retesting later to confirm.
Prevention Beyond Condoms
Condoms remain one of the most accessible and effective barriers against fluid-borne STIs, but they are only one layer of protection. Vaccines now exist for three sexually transmitted infections: HPV, hepatitis A, and hepatitis B. The CDC considers pre-exposure vaccination one of the most effective methods for preventing these infections.
For HIV specifically, PrEP provides a pharmaceutical option for people at elevated risk. When taken consistently, it is highly effective at preventing HIV acquisition. People living with HIV who take antiretroviral therapy and maintain an undetectable viral load also eliminate the risk of sexual transmission, a principle often summarized as “undetectable equals untransmittable.”
Regular screening is itself a form of prevention. Because most STIs are asymptomatic, routine testing catches infections early, before they can be passed to partners or progress to complications like PID or infertility. The combined total of reported chlamydia, gonorrhea, and syphilis cases in the U.S. declined 9 percent from 2023 to 2024, marking a third consecutive year of decline, but there were still more than 2.2 million reported cases. Widespread, routine testing remains essential to continuing that trend.
Long-Term Complications of Untreated STIs
The most serious consequences of STIs come from infections that go undetected and untreated. In women, chlamydia and gonorrhea can silently spread to the upper reproductive tract, causing PID. The resulting scarring of the fallopian tubes, uterus, and surrounding tissue can lead to chronic pelvic pain, ectopic pregnancy, and infertility. Because the damage can occur without obvious symptoms, it is sometimes called “silent” infection.
Untreated syphilis can progress over years to affect the cardiovascular system and central nervous system. HPV infections that persist can develop into cancers of the cervix, throat, anus, or penis, often a decade or more after the initial infection. Chronic hepatitis B can lead to cirrhosis and liver cancer. HIV, without treatment, gradually destroys the immune system, leaving the body unable to fight infections that would normally be harmless.
Nearly all of these outcomes are preventable with timely testing, vaccination where available, and appropriate treatment. The gap between what is preventable and what actually happens comes down largely to detection: finding infections before they cause irreversible harm.