An STD, or sexually transmitted disease, is an infection passed between people through sexual contact, including vaginal, oral, and anal sex. The term is used interchangeably with STI (sexually transmitted infection), and health organizations now prefer “STI” because many of these infections cause no symptoms and never progress to what most people would call a “disease.” In the United States alone, more than 2.2 million cases of just three common STIs (chlamydia, gonorrhea, and syphilis) were reported in 2024.
How STIs Spread
STIs spread through the exchange of bodily fluids or direct skin-to-skin contact during sex. The organisms responsible, whether bacteria, viruses, or parasites, enter the body through microscopic breaks in the moist tissue lining the genitals, mouth, or rectum. Some infections like herpes and HPV spread primarily through skin contact, while others like chlamydia and gonorrhea travel through fluids such as semen, vaginal secretions, or blood.
Sexual intercourse isn’t the only route. STIs can also pass from a pregnant person to their baby during childbirth or breastfeeding. Hepatitis B and hepatitis C can spread through shared needles. And while heavy petting or other intimate physical contact can occasionally transmit certain infections, this is uncommon.
Several factors raise the likelihood of getting an STI: having unprotected sex with multiple partners, a prior history of STIs, using alcohol or drugs that lower inhibitions around safer sex, or having a partner who has concurrent sexual relationships. Male circumcision lowers the risk of acquiring several STIs, reducing HIV transmission risk by 50% to 60%.
Most STIs Have No Symptoms
One of the most important things to understand about STIs is that the majority of infected people feel perfectly fine. An estimated 77% of chlamydia cases and 45% of gonorrhea cases never produce noticeable symptoms. This means someone can carry and transmit an infection for months or years without knowing it, which is the main reason regular testing matters.
When symptoms do appear, they vary by infection but tend to follow a few common patterns: unusual discharge from the penis or vagina, pain or burning during urination, sores or bumps on the genitals, and pelvic or lower abdominal pain. Here’s what the most common STIs look like when they do cause symptoms.
Chlamydia and Gonorrhea
These two bacterial infections overlap in how they feel. Both can cause painful urination, abnormal discharge, and pain in the lower abdomen or pelvis. Gonorrhea tends to produce thicker, cloudier discharge that may be bloody. Both can infect the rectum, causing pain, discharge, or bleeding there. In women, either infection can trigger bleeding between periods. In men, swollen or painful testicles are a possible sign.
Genital Herpes
Herpes typically shows up as small red bumps, blisters, or open sores around the genitals, rectum, or mouth. The area often itches or hurts before the sores appear. Outbreaks tend to recur, especially in the first year, though they usually become less frequent over time. Between outbreaks, the virus lies dormant but can still be transmitted.
HPV and Genital Warts
Human papillomavirus is the most common STI overall. Most HPV infections clear on their own without symptoms. When warts do develop, they appear as small bumps in the genital area, sometimes clustering into a cauliflower-like shape. Other strains of HPV don’t cause warts but can lead to cervical, throat, or anal cancer years later.
Syphilis
Syphilis progresses through distinct stages. It begins with a single painless, firm, round sore (called a chancre) wherever the bacteria entered the body. Because it doesn’t hurt, it’s easy to miss. If untreated, a second stage follows weeks later with a rough, non-itchy rash that can cover the palms and soles, along with fever, swollen lymph nodes, fatigue, sore throat, and patchy hair loss. Left untreated for years, syphilis can damage the brain, heart, and other organs.
Bacterial vs. Viral: What’s Curable
STIs caused by bacteria or parasites, including chlamydia, gonorrhea, syphilis, and trichomoniasis, are curable with antibiotics. A full course of treatment clears the infection, though you can be reinfected if exposed again.
Viral STIs, including herpes, HPV, HIV, and hepatitis B, have no cure. Antiviral medications can manage symptoms, reduce the frequency of outbreaks, and significantly lower the chance of transmitting the virus to a partner. With modern treatment, HIV can be suppressed to undetectable levels, meaning it won’t progress and effectively can’t be passed on sexually. HPV infections often clear naturally within a year or two, though the cancer-causing strains require monitoring.
What Happens If STIs Go Untreated
Untreated bacterial STIs can cause serious, sometimes irreversible damage. In women, chlamydia and gonorrhea can travel upward into the uterus and fallopian tubes, causing pelvic inflammatory disease (PID). PID leads to chronic pelvic pain and scarring that can block the fallopian tubes, resulting in infertility or ectopic pregnancy, a dangerous condition where a fertilized egg implants outside the uterus. Early treatment of PID significantly lowers the risk of these outcomes.
Herpes raises the risk of acquiring HIV. Among people with herpes, up to 50% of HIV infections may be attributable to the herpes infection itself, because the genital sores create an entry point for the virus. Untreated syphilis can cause neurological damage, blindness, and heart problems. And STIs in pregnant women can cause serious complications for newborns, including congenital infections and preterm birth.
Testing and Window Periods
Because most STIs don’t cause symptoms, testing is the only reliable way to know your status. How soon a test can detect an infection after exposure varies by pathogen. Chlamydia and gonorrhea can be detected almost immediately after exposure through urine samples or swabs. Syphilis requires at least four weeks to show up on a blood test. HIV needs at least six weeks, though newer tests are narrowing this window. Herpes testing is most accurate when done on an active sore; the longer you wait after a sore appears, the less reliable the swab becomes.
If you’ve been exposed to an STI, timing your test correctly prevents a false negative. Testing too early can miss an infection that’s still developing.
Prevention
Condoms remain the most accessible and effective barrier against STIs transmitted through fluids, though they’re less protective against infections spread through skin contact, like herpes and HPV, because they don’t cover all potentially infected skin.
Vaccination provides strong protection against two major viral STIs. The HPV vaccine is given as a two-dose series for people who start between ages 9 and 14, or a three-dose series for those starting at age 15 or older (up to age 45). Hepatitis B vaccination is part of the routine childhood immunization schedule and is also available for unvaccinated adults.
A newer prevention tool is post-exposure antibiotic prophylaxis, currently recommended by the CDC for men who have sex with men and transgender women who’ve had a bacterial STI in the past year. Taken within 72 hours after sex, it reduced chlamydia infections by roughly 70% to 88% and syphilis infections by roughly 73% to 87% in clinical trials. This approach is not yet recommended for other populations, as a trial in cisgender women in Kenya found no significant benefit.
How Common STIs Are in the U.S.
Chlamydia is by far the most frequently reported STI in the United States, with nearly 1.52 million cases in 2024. Gonorrhea followed at about 543,000 cases, and syphilis at roughly 190,000. The combined total of these three infections dropped 9% from 2023, marking a third consecutive year of decline, but the numbers remain substantial. These figures also only reflect reported cases. Given the high rate of asymptomatic infections, the true number of people living with an STI at any given time is considerably higher.