There are more than 30 known sexually transmitted infections, but eight account for the vast majority of cases worldwide. Four of those are bacterial and curable: chlamydia, gonorrhea, syphilis, and trichomoniasis. The other four are viral and currently not curable, though they are treatable: HIV, HPV, herpes (HSV), and hepatitis B. Beyond these major eight, several less common infections also spread through sexual contact, including parasites, fungi, and even some gut pathogens.
In the United States alone, more than 2.2 million cases of just chlamydia, gonorrhea, and syphilis were reported in 2024. That number doesn’t include viral infections like herpes and HPV, which are far more prevalent but aren’t tracked the same way. Here’s a breakdown of every significant STI, how it spreads, what it does, and what to know about testing.
STI vs. STD: A Quick Distinction
You’ll see both terms used interchangeably, but they mean slightly different things. An STI is the initial infection, a virus, bacterium, or parasite you pick up through sexual contact. An STD is what develops when that infection causes noticeable symptoms or disease. Many STIs never progress to that point. Someone can carry and transmit chlamydia or HPV for months or years without any symptoms at all. Health agencies now prefer “STI” because it’s more accurate for the millions of cases that are silent.
Bacterial STIs (All Curable)
Chlamydia
Chlamydia is the most commonly reported STI in the U.S., with roughly 1.5 million cases in 2024. It spreads through vaginal, anal, or oral sex when bodily fluids contact mucous membranes. Most people with chlamydia have no symptoms at all, which is why routine screening matters. When symptoms do appear, they typically show up one to three weeks after exposure and can include unusual discharge, burning during urination, or rectal pain. Left untreated, chlamydia can cause pelvic inflammatory disease and fertility problems. It’s cured with a short course of antibiotics.
Gonorrhea
Gonorrhea had over 543,000 reported cases in the U.S. in 2024. It infects the same sites as chlamydia (genitals, rectum, throat) and often co-occurs with it. Symptoms, when present, tend to appear within two to eight days: thick discharge, painful urination, or a sore throat if the infection is oral. Vaginal infections are frequently asymptomatic. Gonorrhea is curable, but antibiotic-resistant strains have become a growing concern worldwide.
Syphilis
Syphilis is caused by a spiral-shaped bacterium and progresses through distinct stages if untreated. The first sign is typically a painless sore (called a chancre) at the site of infection, appearing around three weeks after exposure, though it can take up to 90 days. Because the sore is painless and sometimes hidden inside the body, many people miss it entirely. Without treatment, syphilis moves into a second stage with rashes and flu-like symptoms, then goes silent for years before potentially causing serious damage to the heart, brain, and other organs. 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 is fully curable, especially when caught early. Testing requires a blood draw, and it can take one to three months after exposure for results to be reliable.
Mycoplasma Genitalium
This is a lesser-known bacterial STI that has gained attention in recent years. It can cause urethritis in men and cervicitis or pelvic inflammatory disease in women, but it often produces no symptoms. It’s not part of standard STI panels in most clinics, so it tends to be tested for only when symptoms persist after treatment for chlamydia or gonorrhea. It responds to antibiotics, though some strains are becoming resistant.
Chancroid and LGV
Chancroid causes painful genital ulcers and is most common in tropical regions. It’s rare in the U.S. and other industrialized countries. Lymphogranuloma venereum (LGV) is caused by specific strains of the same bacterium behind chlamydia. It most commonly causes rectal inflammation and symptoms like bloody discharge and constipation. LGV has been seen in outbreaks primarily among men who have sex with men. Both are treatable with antibiotics.
Viral STIs (Treatable, Not Curable)
Human Papillomavirus (HPV)
HPV is the single most common STI. Most sexually active people will contract at least one strain at some point in their lives. There are over 100 strains, and the vast majority clear on their own within a year or two without causing problems. Some strains cause genital or anal warts, which can appear weeks to many months after exposure. Other “high-risk” strains can, over years, lead to cervical, throat, anal, or penile cancers. There’s no routine screening test for HPV in men. For women, HPV is detected through Pap smears, with reliable results appearing a few weeks to a few months after infection. The HPV vaccine is highly effective at preventing the strains responsible for most cancers and genital warts. HPV spreads through skin-to-skin genital contact, meaning condoms reduce but don’t eliminate risk.
Herpes Simplex Virus (HSV)
Herpes comes in two types. HSV-1 traditionally causes oral cold sores but increasingly causes genital infections through oral sex. HSV-2 is the classic genital herpes strain. Both spread through skin-to-skin contact, and both can transmit even when no sores are visible. Symptoms, if they occur, usually appear within two to twelve days as painful blisters or sores that heal and then recur periodically. Many people with herpes have symptoms so mild they never realize they’re infected. There’s no cure, but antiviral medications reduce outbreaks and lower the chance of transmission. Blood tests for herpes antibodies take one to four months after exposure to become accurate.
HIV
HIV attacks the immune system and, without treatment, progresses to AIDS. It spreads through bodily fluids: blood, semen, vaginal fluids, and breast milk. It does not spread through skin-to-skin contact alone. Early infection sometimes causes flu-like symptoms (fever, body aches) within one to two weeks, then the virus can remain silent for years while slowly weakening the immune system. Modern antiretroviral therapy allows people with HIV to live long, healthy lives and can reduce the virus to undetectable levels where it cannot be sexually transmitted. Blood-based antigen/antibody tests can detect HIV as early as two weeks after exposure, with most infections caught by six weeks. Oral swab tests take longer, up to three months for full reliability.
Hepatitis B
Hepatitis B is a viral liver infection that spreads through blood, semen, and other bodily fluids. Many people have no symptoms initially. When symptoms appear, they typically emerge around six weeks after exposure and include fatigue, nausea, abdominal pain, and jaundice. Most adults clear the infection on their own, but about 5% develop chronic hepatitis B, which can lead to liver damage or liver cancer over time. A highly effective vaccine exists and is part of the standard childhood immunization schedule. Blood tests detect the virus within three to six weeks.
Hepatitis C
Hepatitis C primarily spreads through blood-to-blood contact, making it less commonly classified as an STI. However, sexual transmission does occur, particularly during activities involving blood exposure. Most people have no symptoms for weeks or months. It’s now curable in the vast majority of cases with antiviral medications, a major advance from just a decade ago. Antibody blood tests become reliable about two months after exposure, though it can take up to six months in some cases.
Parasitic STIs
Trichomoniasis
Trichomoniasis is caused by a single-celled parasite and is the most common curable STI worldwide. It primarily infects the vagina and urethra. Symptoms, when they appear, develop within five to 28 days and include itching, burning, unusual discharge (sometimes frothy or yellow-green), and discomfort during urination or sex. Many infections are asymptomatic, particularly in men. It’s diagnosed with a simple swab test and cured with a single dose of oral medication. Testing is reliable about a week after exposure.
Pubic Lice and Scabies
Pubic lice (“crabs”) are tiny insects that attach to coarse body hair, primarily in the genital area. They cause intense itching that starts within days to two weeks of exposure. Scabies is caused by a microscopic mite that burrows into the skin, producing a characteristic itchy rash and tiny raised lines (burrows), especially in the finger webs and genital area. Both spread through close physical contact and are treated with topical medications. Neither requires a lab test for diagnosis; they’re identified by visual examination.
Intestinal Parasites
Oral-anal contact can transmit gut parasites like those causing amebiasis and giardiasis. Both can cause acute or chronic diarrhea, cramping, and other digestive symptoms. They’re diagnosed through stool samples and treated with antiparasitic medication. These infections are more commonly associated with contaminated food or water but are well-documented as sexually transmitted, particularly among men who have sex with men.
How Different STIs Spread
Not all STIs spread the same way, and this distinction matters for protection. Some require the exchange of bodily fluids, while others pass through skin-to-skin contact alone.
STIs that spread through skin-to-skin genital contact, even without penetration, include herpes, HPV, syphilis, and mpox. Condoms help but can’t fully prevent these because the infected skin may not be covered.
STIs that primarily spread through bodily fluids (semen, vaginal fluids, blood) include chlamydia, gonorrhea, and HIV. These require contact between fluids and mucous membranes or breaks in the skin. Condoms are highly effective against these when used consistently.
Blood-borne STIs like HIV, hepatitis B, and hepatitis C can also transmit through shared needles or any activity involving blood contact with open skin.
Testing Windows by Infection
Getting tested too soon after exposure can produce a false negative. Each STI has a “window period,” the time it takes for a test to reliably detect the infection. Here’s a practical guide:
- Chlamydia and gonorrhea: One week catches most cases; two weeks catches nearly all.
- Syphilis: One month catches most; three months catches nearly all.
- HIV (blood test): Two weeks catches most; six weeks catches nearly all.
- Herpes (blood test): One month catches most; four months catches nearly all.
- Trichomoniasis: One week catches most; one month catches nearly all.
- Hepatitis B: Three to six weeks.
- Hepatitis C: Two months catches most; six months catches nearly all.
- HPV (Pap smear): Three weeks to a few months.
Standard STI panels typically test for chlamydia, gonorrhea, syphilis, and HIV. Herpes, HPV, hepatitis, trichomoniasis, and less common infections usually require specific requests. If you’ve had a particular type of exposure that concerns you, mention it so the right tests get ordered.