There are more than 20 sexually transmitted infections recognized by health authorities, ranging from well-known ones like chlamydia and HIV to lesser-known infections like mycoplasma genitalium and donovanosis. In the United States alone, over 2.2 million cases of just three STIs (chlamydia, gonorrhea, and syphilis) were reported in 2024. Here’s a breakdown of the major types, organized by what causes them.
Bacterial STIs
1. Chlamydia is the most commonly reported STI in the U.S., with roughly 1.5 million cases in 2024. It often causes no symptoms at all, which is why routine screening matters. When symptoms do appear, they include burning during urination and unusual discharge. Left untreated, chlamydia can lead to pelvic inflammatory disease and fertility problems. It’s curable with antibiotics.
2. Gonorrhea accounted for about 543,000 reported cases in 2024. Men are more likely to notice symptoms like painful urination and discharge, while women often have no recognizable symptoms until complications develop. Gonorrhea is treatable, but the bacteria have developed resistance to many antibiotics over the years, making effective treatment increasingly important to get right on the first attempt.
3. Syphilis progresses through distinct stages. The first sign is typically a painless sore (called a chancre) at the site of infection. If untreated, it advances to a second stage marked by skin rashes and swollen lymph nodes, then can go dormant for years before causing serious damage to the brain, heart, and other organs. About 190,000 total syphilis cases were reported in the U.S. in 2024. It’s curable with penicillin.
4. Mycoplasma genitalium (Mgen) is a newer addition to the STI landscape and often flies under the radar. Most people with Mgen have no symptoms. Those who do may notice discharge from the penis or vagina and burning during urination. It can be diagnosed through a urine sample or swab test.
5. Chancroid causes deep, painful genital ulcers, sometimes accompanied by swollen, tender lymph nodes in the groin. It’s rare in the U.S. but more common in parts of Africa and the Caribbean. It’s curable with antibiotics.
6. Donovanosis (granuloma inguinale) produces painless but distinctive ulcers on the genitals that look beefy red and bleed easily. These sores grow slowly over weeks. It’s also rare in developed countries and responds to antibiotic treatment, though courses can last three weeks or longer.
7. Lymphogranuloma venereum (LGV) is caused by specific strains of the same bacteria behind chlamydia. It can cause genital ulcers, swollen lymph nodes (usually on one side), or inflammation of the rectum. LGV requires a longer course of antibiotics than standard chlamydia.
Viral STIs
8. Human papillomavirus (HPV) is the most common STI overall. It spreads through skin-to-skin contact and has over 100 strains, some of which cause genital warts while others can lead to cervical, throat, and anal cancers. A vaccine is available and highly effective when given before exposure. Most HPV infections clear on their own within two years, but the high-risk strains that persist are responsible for nearly all cervical cancers.
9. Genital herpes (HSV-2) causes recurring outbreaks of painful blisters or sores on or around the genitals. The first outbreak is usually the worst, and subsequent ones tend to become less frequent over time. There’s no cure, but antiviral medication reduces the frequency and severity of outbreaks and lowers the risk of passing it to partners.
10. Oral herpes (HSV-1) primarily causes cold sores around the mouth but can also be transmitted to the genitals through oral sex. A large percentage of the global population carries HSV-1, and many people were infected during childhood through non-sexual contact.
11. HIV attacks the immune system, and without treatment, it progresses to AIDS. Modern antiretroviral therapy allows people with HIV to live long, healthy lives and reduces the viral load to undetectable levels, which effectively prevents sexual transmission. A blood test using antigen/antibody methods can detect HIV as early as two weeks after exposure, though testing at six weeks catches almost all infections.
12. Hepatitis B affects the liver and can become a chronic, lifelong infection. It spreads through sexual contact, shared needles, and from mother to child during birth. A vaccine has been available for decades and is part of the standard childhood immunization schedule. Blood tests can detect it about three to six weeks after exposure.
13. Hepatitis C is primarily spread through blood-to-blood contact, but sexual transmission can occur, particularly during activities that involve bleeding or mucosal tears. It can now be cured in most cases with antiviral medication. Testing is reliable around two months after exposure, though it can take up to six months to detect all cases.
14. Hepatitis A spreads through the fecal-oral route, which includes certain sexual activities. It causes an acute liver infection but doesn’t become chronic. A vaccine is available and widely recommended.
15. Molluscum contagiosum is caused by a poxvirus and produces small, firm, dome-shaped bumps on the skin, often with a dimple in the center. The bumps are usually white, pink, or skin-colored and range from pinhead to pencil-eraser size. In adults, the infection is frequently spread through sexual contact. Bumps appearing near the genitals should be evaluated to rule out other conditions. Molluscum often clears on its own, though treatment can speed the process.
16. Mpox was added to STI surveillance lists after the 2022 outbreak demonstrated significant sexual transmission. It causes a rash that progresses to raised bumps and fluid-filled blisters, often on or near the genitals. A vaccine is available for people at higher risk.
Parasitic STIs
17. Trichomoniasis is caused by a microscopic parasite and is one of the most common curable STIs. Symptoms appear 5 to 28 days after infection, though they can show up later or not at all. Women may notice thin, frothy, foul-smelling discharge that can be clear, white, yellow, or green, along with genital itching and pain during urination or sex. Men rarely have symptoms, but when they do, they may feel itching or irritation inside the penis. The parasite passes between people during genital contact, even when no symptoms are present. It’s easily treated with oral medication.
18. Pubic lice (crabs) are tiny insects that attach to coarse body hair, particularly in the genital area. They cause intense itching, and you can sometimes see the lice or their eggs (nits) attached to hair shafts. They spread through close body contact and can be treated with over-the-counter medicated shampoos.
19. Scabies is caused by microscopic mites that burrow into the skin, causing intense itching that’s often worse at night. While scabies isn’t exclusively sexual, prolonged skin-to-skin contact during sex is a common route of transmission. It’s treated with prescription topical creams.
Sexually Transmitted Gut Infections
20. Shigella is a bacterial gut infection that has become increasingly recognized as sexually transmitted, particularly through oral-anal contact. It causes diarrhea (sometimes bloody), stomach cramps, and fever.
Two additional parasitic gut infections also spread through sexual activity. Giardiasis, caused by the parasite Giardia, transmits exclusively through the fecal-oral route and can cause diarrhea, gas, and stomach cramps. Prevalence rates of 4 to 18% have been documented in studies of men who have sex with men. Amebiasis, caused by the parasite Entamoeba histolytica, is the most common intestinal parasite found in gay communities worldwide, though most people carrying it have no symptoms. Both infections can result from oral-anal contact or oral sex following anal contact.
Conditions Caused by STIs
Some conditions aren’t infections themselves but result from STIs. Pelvic inflammatory disease (PID) is an infection and inflammation of the upper reproductive tract in women, often triggered by untreated chlamydia or gonorrhea. It can cause chronic pelvic pain and damage the fallopian tubes, leading to infertility or ectopic pregnancy. Bacterial vaginosis, while not always sexually transmitted, results from an imbalance in the vaginal microbiome and is more common in sexually active women.
Testing Windows to Know
Getting tested at the right time matters. Test too early and you risk a false negative. HIV blood tests (antigen/antibody method) catch most infections at two weeks, with near-complete accuracy at six weeks. Syphilis blood tests catch most cases at one month, with three months needed to be thorough. Hepatitis B shows up on tests at three to six weeks, while hepatitis C needs about two months for most cases and up to six months to rule out completely. Chlamydia and gonorrhea can generally be detected within one to two weeks of exposure through urine or swab tests.
If you’ve had a potential exposure, testing once at the appropriate window and again at the outer limit gives you the most reliable results. Many STIs have no symptoms at all, which is why screening based on risk factors rather than symptoms catches infections that would otherwise go unnoticed and untreated.