Sexually transmitted diseases (STDs) fall into four main categories based on what causes them: bacteria, viruses, parasites, and fungi. There are dozens of distinct infections, but roughly ten account for the vast majority of cases. In 2024 alone, more than 2.2 million cases of just chlamydia, gonorrhea, and syphilis were reported in the United States. Understanding the different types helps you recognize symptoms, get the right test, and know what to expect from treatment.
Bacterial STDs
Bacterial STDs are caused by bacteria and are generally curable with antibiotics, often in a single dose. The three most common are chlamydia, gonorrhea, and syphilis.
Chlamydia is the most frequently reported STD in the U.S., with roughly 1.5 million cases in 2024. Most people with chlamydia have no symptoms at all, which is why routine screening matters. When symptoms do appear, they typically include unusual discharge, burning during urination, or pain during sex. Left untreated, chlamydia can lead to pelvic inflammatory disease (PID) in women, which causes scarring in the fallopian tubes. About 1 in 8 women with a history of PID have difficulty getting pregnant.
Gonorrhea accounted for about 543,000 reported cases in 2024. It shares many symptoms with chlamydia, and the two infections often occur together. Gonorrhea can infect the genitals, rectum, and throat, and like chlamydia, it frequently causes no noticeable symptoms. Untreated gonorrhea can also cause PID and may spread to the bloodstream or joints in rare cases.
Syphilis progresses through distinct stages if left untreated, making it unique among bacterial STDs. In the primary stage, a firm, round, painless sore appears where the bacteria entered the body. Because the sore doesn’t hurt, many people never notice it. It heals on its own within three to six weeks, but the infection hasn’t gone away. In the secondary stage, a rash develops, often on the palms of the hands or soles of the feet, sometimes accompanied by fever, hair loss, and fatigue. After that, syphilis enters a latent stage with no visible symptoms that can last years. Tertiary syphilis, which can appear 10 to 30 years after the initial infection, damages internal organs and can be fatal. About 190,000 total syphilis cases were reported in 2024, and congenital syphilis (passed from mother to baby during pregnancy) has been rising for 12 consecutive years.
Two other bacterial STDs worth knowing about: Mycoplasma genitalium (Mgen), which causes symptoms similar to chlamydia and is increasingly recognized as a common cause of urethritis, and bacterial vaginosis (BV), a disruption in vaginal bacteria that can be triggered by sexual activity.
Viral STDs
Viral STDs are caused by viruses and, unlike bacterial infections, cannot be cured with antibiotics. Some can be managed with medication, and one (HPV) has an effective vaccine. The major viral STDs are HPV, herpes, HIV, and hepatitis B and C.
Human papillomavirus (HPV) is the most common STD overall. Most HPV infections clear on their own without causing problems, but 12 high-risk strains can lead to cancer. HPV can cause six types of cancer: cervical, anal, oropharyngeal (back of the throat), penile, vaginal, and vulvar. Low-risk strains cause genital warts. The HPV vaccine is highly effective at preventing infection from the most dangerous strains and is recommended before a person becomes sexually active.
Genital herpes is caused by herpes simplex virus, most commonly type 2 (HSV-2), though type 1 (which also causes cold sores) can infect the genitals. Herpes causes recurring outbreaks of painful blisters or sores, though many people carry the virus without ever having noticeable symptoms. There is no cure, but antiviral medication reduces the frequency and severity of outbreaks and lowers the risk of transmission.
HIV attacks the immune system and, without treatment, progresses to AIDS. Modern antiretroviral therapy allows people with HIV to live long, healthy lives and can reduce the amount of virus in the blood to undetectable levels, which also prevents sexual transmission. HIV is spread through blood, semen, vaginal fluids, and breast milk.
Hepatitis B and C are liver infections that can be transmitted sexually, though hepatitis C is more commonly spread through blood-to-blood contact. Hepatitis B has an effective vaccine. Both can become chronic and lead to liver damage or liver cancer if untreated.
Parasitic STDs
Trichomoniasis is the most common curable STD worldwide and is caused by a single-celled parasite. In women, it typically causes vaginitis: itching, burning, unusual discharge that may be yellowish-green and frothy, and discomfort during urination. Many men with trichomoniasis have no symptoms. It’s curable with a single course of antiparasitic medication.
Pubic lice (sometimes called crabs) are tiny insects that attach to coarse body hair, primarily in the genital area. The main symptom is intense itching. You or a healthcare provider can diagnose them by spotting the lice or their eggs (nits) on the hair. Scabies is caused by a microscopic mite that burrows into the skin, creating characteristic linear tracks and a papular rash, especially in the finger webs and genital area. Both pubic lice and scabies are treated with topical medications and are fully curable.
Why Most STDs Go Unnoticed
The majority of STDs are asymptomatic, meaning they cause no obvious symptoms. This is true across categories. Chlamydia, gonorrhea, HPV, herpes, and even early syphilis can all be present without any signs. The World Health Organization estimates that more than one million curable STIs are acquired every day worldwide, and most of them produce no symptoms at all. This is why STDs spread so efficiently: people pass them on without realizing they’re infected.
Routine screening is the only reliable way to catch these infections early. You can’t tell whether someone has an STD by looking at them, and you can’t assume you’re infection-free just because you feel fine.
Testing Windows for Common STDs
Every STD has a window period, the time between exposure and when a test can reliably detect the infection. Testing too early can produce a false negative. Here’s what to expect for the most common STDs:
- Chlamydia and gonorrhea: Detected by urine sample or swab. One week catches most infections; two weeks catches nearly all.
- Syphilis: Detected by blood test. One month catches most; three months catches nearly all.
- HIV: A blood test using antigen/antibody methods detects most infections within two weeks and nearly all by six weeks. Oral swab tests take longer, up to three months for full accuracy.
- Herpes: Detected by blood test (antibody). One month catches most; four months catches nearly all.
- Trichomoniasis: Detected by vaginal swab. One week catches most; one month catches nearly all.
- Hepatitis B: Detected by blood test at three to six weeks.
- Hepatitis C: Detected by blood test. Two months catches most; six months catches nearly all.
- HPV: Detected in women through a Pap smear, typically three weeks to a few months after exposure. There is no approved screening test for HPV in men.
There are no screening tests for genital warts, molluscum contagiosum, or pubic lice. These are diagnosed visually when symptoms appear.
Treatment by Category
The type of organism behind an STD determines whether it can be cured or only managed. Bacterial STDs (chlamydia, gonorrhea, syphilis) and parasitic STDs (trichomoniasis, pubic lice, scabies) are all curable, often with a single dose or short course of medication. The key is catching them early. The longer a bacterial STD goes untreated, the higher the risk of complications like PID, infertility, ectopic pregnancy, and chronic pelvic pain.
Viral STDs cannot be cured, but most can be effectively managed. Herpes outbreaks can be controlled with antiviral medication. HIV is managed with daily antiretroviral therapy that keeps the virus suppressed. Hepatitis B and C have specific treatments, and hepatitis C can now be cured in most cases with a course of antiviral pills lasting 8 to 12 weeks. HPV infections usually resolve on their own, but the cancers they cause require separate treatment, which is why vaccination and regular screening (like Pap smears) are so important.
What Raises Your Risk
Several factors increase the likelihood of getting an STD. Having unprotected sex is the most significant. Having multiple sexual partners, or a partner who has multiple partners, raises exposure risk. Being under 25 is a risk factor because STD rates are highest in younger age groups. Having one STD also makes it easier to contract another, particularly HIV, because open sores or inflammation give the virus easier access to the bloodstream.
Condom use substantially reduces the risk of most STDs, though it doesn’t eliminate it entirely for infections spread through skin-to-skin contact, like herpes and HPV. Vaccination is available for HPV and hepatitis B. Pre-exposure prophylaxis (PrEP) is highly effective at preventing HIV in people at elevated risk.