Sexually transmitted diseases (STDs) are infections passed between people through vaginal, anal, or oral sex. They fall into three categories based on what causes them: bacteria, viruses, or parasites. In the United States alone, over 2.2 million cases of chlamydia, gonorrhea, and syphilis were reported in 2024, and those are just the ones that get diagnosed. Many STDs produce no symptoms at all, which is why they spread so easily and why understanding what they are matters.
The terms “STD” and “STI” (sexually transmitted infection) are often used interchangeably, but there’s a technical difference. An STI becomes an STD when the infection starts causing noticeable symptoms or damage. Since many sexually transmitted infections never reach that point, health experts increasingly prefer “STI” because “disease” implies an obvious medical problem that isn’t always there.
Bacterial STIs
Bacterial STIs are caused by bacteria and can be cured with antibiotics. The three most common are chlamydia, gonorrhea, and syphilis.
Chlamydia is the most frequently reported bacterial STI in the U.S., with roughly 1.5 million cases in 2024. An estimated 77% of chlamydia cases never produce symptoms. When symptoms do appear, they typically involve unusual discharge from the penis or vagina, burning during urination, or lower abdominal pain. Left untreated, chlamydia can cause pelvic inflammatory disease and fertility problems.
Gonorrhea accounted for about 543,000 reported cases in 2024. Like chlamydia, it often flies under the radar: roughly 45% of gonorrhea infections never cause symptoms. When they do, expect discharge, painful urination, or soreness. Gonorrhea can infect the throat and rectum as well as the genitals, and untreated infections can lead to serious reproductive damage and, rarely, spread to the bloodstream.
Syphilis is less common but more dangerous if ignored. There were over 190,000 total syphilis cases reported in 2024, including nearly 4,000 cases of congenital syphilis (passed from mother to baby during pregnancy). Syphilis progresses through four distinct stages:
- Primary: A firm, round, painless sore appears where the bacteria entered the body, often on the genitals, anus, or mouth. Because it doesn’t hurt, many people miss it entirely. The sore heals on its own in three to six weeks, but the infection remains.
- Secondary: A rash develops, sometimes on the palms of the hands or soles of the feet, along with fever, swollen lymph nodes, sore throat, patchy hair loss, and fatigue.
- Latent: All visible symptoms disappear. Without treatment, the bacteria stay in the body for years with no outward signs.
- Tertiary: This stage can occur 10 to 30 years after the initial infection, damaging the heart, blood vessels, brain, and nervous system. It can be fatal. Most people with untreated syphilis don’t reach this stage, but some do.
Viral STIs
Viral STIs are caused by viruses. Unlike bacterial STIs, they cannot be cured, though antiviral medications can manage symptoms and reduce the risk of spreading the infection.
HPV (human papillomavirus) is the most common STI in the United States. There are many different types of HPV. Most clear on their own without causing problems, but certain strains cause genital warts, and others can lead to cervical, throat, and anal cancers over time. A vaccine is available and highly effective at preventing the most dangerous strains.
Herpes comes in two forms. HSV-1 typically causes oral cold sores, while HSV-2 is more commonly associated with genital sores, though either type can infect either location. Symptoms, when they appear, usually show up within about 12 days of exposure as small red bumps or blisters around the genitals, rectum, or mouth. These sores heal in a week or longer. For many people, outbreaks come and go over years, often becoming less frequent with time.
HIV attacks the immune system. About two to four weeks after infection, most people experience flu-like symptoms: fever, fatigue, swollen lymph nodes. These pass, and the virus enters a chronic phase that can last years with few obvious signs, though weight loss, diarrhea, and persistent swollen lymph nodes may develop. Without treatment, HIV progresses to AIDS. With modern antiviral therapy taken as prescribed, many people with HIV maintain a normal life expectancy and can stay in the chronic phase indefinitely.
Hepatitis B is a viral liver infection that spreads through sexual contact, shared needles, and from mother to child during birth. It can become chronic and lead to serious liver damage. A vaccine exists and is routinely given in childhood.
Parasitic STIs
Trichomoniasis is the most common nonviral STI worldwide, affecting an estimated 2.6 million people in the United States. It’s caused by a tiny parasite. Between 70% and 85% of infected people have minimal or no symptoms, and untreated infections can persist for months or even years. When symptoms do develop, women may notice yellow-green vaginal discharge with an unpleasant odor and vulvar irritation. Men may experience urethral discomfort. Trichomoniasis is curable with a short course of antibiotics.
Pubic lice (sometimes called “crabs”) are another parasitic STI, spread through close body contact. They cause intense itching in the genital area and are treatable with over-the-counter medicated shampoos.
How STIs Spread
STIs spread through contact with infected body fluids or skin. Vaginal, anal, and oral sex are the primary routes. The organisms enter the body through tiny, often invisible breaks in the mucous membranes lining the genitals, anus, and mouth.
Not all STIs spread the same way. HIV, gonorrhea, chlamydia, and hepatitis B spread mainly through fluid exchange (semen, vaginal fluids, blood). Herpes, HPV, and syphilis spread through direct skin-to-skin contact, which means condoms reduce the risk but don’t eliminate it for these infections, since sores or infected skin outside the area covered by a condom can still transmit the virus or bacteria. Some STIs can also pass from mother to baby during childbirth or breastfeeding, and a few (like HIV and hepatitis) can spread through shared needles.
Why Most People Don’t Know They Have One
The single most important thing to understand about STIs is that most of them produce no symptoms. About 77% of chlamydia infections, 45% of gonorrhea infections, and 70 to 85% of trichomoniasis cases are completely silent. Herpes can lie dormant for long stretches. HIV can take years to cause noticeable problems. This means someone can carry and transmit an infection without ever feeling sick, which is why routine testing is the only reliable way to know your status.
When symptoms do appear, the most common signs across STIs include unusual discharge from the penis or vagina, painful or frequent urination, sores or blisters on the genitals or mouth, itching or redness in the genital area, abnormal vaginal odor, and abdominal pain.
How STIs Are Detected
Different STIs require different tests. Blood tests are used for syphilis, HIV, hepatitis B, and sometimes herpes. Urine tests can detect chlamydia, gonorrhea, and trichomoniasis. Swab tests, where a sample is taken from the vagina, penis, rectum, or a sore, are used for HPV, chlamydia, gonorrhea, and herpes. Some infections, like genital warts and pubic lice, are diagnosed by visual examination alone.
Timing matters. Each STI has a “window period,” the gap between exposure and when a test can reliably detect it. Chlamydia and gonorrhea can usually be caught on a test within one to two weeks. Syphilis takes about a month to show up reliably, with three months needed to catch nearly all cases. HIV can be detected via blood test in as little as two weeks, but a full six weeks is recommended for the most accurate result. Herpes antibodies take one to four months to develop. If you test too early after a possible exposure, a negative result may not be accurate.
How Condoms and Vaccines Help
Condoms are the most accessible form of STI prevention, but their effectiveness varies significantly depending on the infection. For STIs transmitted primarily through fluids, condoms work well: they reduce HIV transmission by about 85% with consistent use and offer over 90% protection against hepatitis B and gonorrhea. For chlamydia, studies show reductions ranging from 33% to nearly 100% depending on how consistently condoms are used.
For STIs that spread through skin contact, condoms are less effective. Syphilis transmission drops by 50 to 71% with perfect use. Genital herpes risk drops by about 40%. HPV protection from condoms is minimal, since the virus can live on skin that a condom doesn’t cover.
Vaccines fill some of those gaps. The HPV vaccine is highly effective at preventing the strains responsible for most HPV-related cancers and genital warts. The hepatitis B vaccine provides strong, long-lasting protection. No vaccines currently exist for chlamydia, gonorrhea, syphilis, herpes, or HIV.
Treatment Overview
Bacterial STIs (chlamydia, gonorrhea, syphilis) and parasitic STIs (trichomoniasis) are curable with antibiotics. Treatment is straightforward, and the infections clear completely. Both you and your sexual partner need to be treated to avoid passing the infection back and forth, and you’ll need to avoid sex for a period after treatment.
Viral STIs (herpes, HIV, hepatitis B, HPV) have no cure, but they’re manageable. Antiviral medications can suppress herpes outbreaks, keep HIV at undetectable levels, and control hepatitis B. Many HPV infections clear on their own within a year or two, though the strains that cause cancer require monitoring. Early detection and treatment of any STI reduces the risk of long-term complications like infertility, organ damage, and certain cancers.