There are more than 25 sexually transmitted infections recognized by health organizations, caused by bacteria, viruses, parasites, and even fungi. The term “STD” and “STI” are often used interchangeably, though medical organizations now prefer “STI” because many of these infections cause no symptoms and never progress to what most people would call a disease. Below is a comprehensive breakdown of the types, organized by what causes them, along with what you should know about each one.
Bacterial STIs
Chlamydia is the most commonly reported STI in the United States, with over 1.5 million cases reported in 2024 alone. It often produces 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 or burning during urination. Left untreated, chlamydia can cause pelvic inflammatory disease in women, potentially leading to infertility.
Gonorrhea ranks second, with about 543,000 cases reported in 2024. Symptoms usually appear within two to eight days but can take up to two weeks. Like chlamydia, it frequently causes no noticeable symptoms, especially in vaginal infections. Gonorrhea has become a growing concern because some strains are developing resistance to common antibiotics.
Syphilis progresses through distinct stages if untreated: a painless sore, then a rash, then a latent phase that can last years, and finally serious damage to the heart, brain, and other organs. About 190,000 total syphilis cases were reported in the U.S. in 2024. The initial sore is often so mild that people miss it entirely. Symptoms typically appear around 21 days after exposure but can take anywhere from 10 to 90 days.
Mycoplasma genitalium (Mgen) is a lesser-known bacterial STI that the CDC now lists alongside the major infections. It can cause urethritis in men and cervicitis or pelvic inflammatory disease in women. It’s increasingly recognized as a common cause of symptoms that don’t test positive for chlamydia or gonorrhea.
Chancroid causes painful genital ulcers and swollen lymph nodes in the groin. It’s rare in the U.S. but more common in parts of Africa, Asia, and the Caribbean.
Lymphogranuloma venereum (LGV) is caused by specific aggressive strains of the same bacterium behind chlamydia. It produces genital ulcers and painful, swollen lymph nodes. Outbreaks have been reported primarily among men who have sex with men.
Donovanosis (granuloma inguinale) causes slowly expanding genital ulcers that bleed easily. It’s extremely rare in developed countries but still occurs in tropical regions.
Bacterial vaginosis (BV) is listed by the CDC as an STI, though its exact transmission dynamics are still debated. It involves an imbalance of bacteria in the vagina, causing fishy-smelling discharge. It’s strongly associated with sexual activity and having new or multiple partners.
Viral STIs
Human papillomavirus (HPV) is the single most common STI overall. Most sexually active people contract at least one strain during their lifetime. There are over 100 strains: some cause genital warts, others cause cervical, throat, and anal cancers, and most clear on their own without ever causing problems. A Pap smear can detect cervical HPV, but the infection can take months to years to become detectable.
Genital herpes is caused by two related viruses: HSV-1 (traditionally associated with cold sores) and HSV-2 (traditionally genital). Both can infect either location. Herpes causes recurring outbreaks of painful blisters, though many people have such mild symptoms they never realize they’re infected. After initial exposure, symptoms can appear in as few as two days or take up to 12. A blood test for herpes antibodies takes about one month to catch most infections and up to four months to catch nearly all of them.
HIV attacks the immune system and, without treatment, progresses to AIDS. Early infection sometimes causes mild flu-like symptoms within one to two weeks, then enters a long latent phase that can last years with no symptoms at all. Modern antiretroviral therapy allows people with HIV to live normal lifespans and reduce viral levels to the point where transmission through sex becomes effectively impossible. A blood-based antigen/antibody test can detect HIV as early as two weeks after exposure, with six weeks catching nearly all cases.
Hepatitis B is a liver infection transmitted through sexual contact, shared needles, and from mother to child during birth. Many people have no symptoms initially, though the incubation period can stretch up to six months. A vaccine is available and highly effective. Testing typically becomes reliable three to six weeks after exposure.
Hepatitis C primarily spreads through blood-to-blood contact but can be transmitted sexually, particularly during rough sex or among people with HIV. Symptoms often don’t appear for weeks to months. A blood antibody test catches most infections by two months, though full confidence requires up to six months. Hepatitis C is now curable with antiviral treatment.
Hepatitis A spreads through oral-fecal contact, which includes certain sexual practices. It causes acute liver inflammation with fatigue, nausea, and jaundice. Unlike hepatitis B and C, it doesn’t become chronic. A vaccine is available.
Molluscum contagiosum produces small, firm, dome-shaped bumps on the skin. In adults, it typically spreads through skin-to-skin sexual contact. The bumps are painless and usually resolve on their own within 6 to 12 months, though they can be removed for cosmetic reasons.
Zika virus can be sexually transmitted, though it spreads more commonly through mosquito bites. The virus persists in semen far longer than in other body fluids, with viral genetic material detectable for an average of 54 days after symptoms begin. The longest documented case of sexual transmission occurred 32 to 41 days after the infected partner’s symptoms started. Zika is primarily a concern for pregnant women because of its link to birth defects.
Ebola can also be transmitted sexually, with the virus persisting in semen for months after recovery. Like Zika, it’s far more commonly spread through other routes.
Parasitic STIs
Trichomoniasis is caused by a microscopic parasite and is one of the most common curable STIs. Many people, especially men, have no symptoms. When symptoms appear (usually 5 to 28 days after exposure), women may notice frothy, foul-smelling vaginal discharge and itching. A single dose of oral antibiotics clears the infection. Testing by vaginal swab catches most infections within one week.
Pubic lice (crabs) are tiny insects that attach to coarse body hair, primarily in the genital area. They spread through sexual contact and cause intense itching. You can sometimes see the lice or their eggs (nits) attached to hair shafts. Over-the-counter treatments are usually effective.
Scabies is a skin infestation caused by a burrowing mite. In adults, it frequently spreads through sexual contact. The hallmark symptom is intense itching, which develops as the body becomes sensitized to the mites. The first time you get scabies, it can take weeks before itching begins. A second infestation triggers itching within 24 hours.
Giardia is an intestinal parasite that can spread through sexual contact involving oral-fecal transmission. It causes diarrhea, gas, and stomach cramps.
Enteric Infections Spread Through Sex
Shigella is a bacterial infection that causes severe diarrhea and can spread easily during sexual activity. The bacteria can be present on skin in the anal and genital areas, including the thighs and buttocks, with no visible signs. Any sexual contact involving these areas, including through shared toys, can transmit the infection.
Cryptosporidium and Entamoeba histolytica (amoebiasis) are additional intestinal parasites that can spread through oral-anal contact during sex.
Conditions Caused by STIs
Several conditions aren’t infections themselves but result directly from untreated STIs. These round out the broader picture of sexually transmitted diseases.
Pelvic inflammatory disease (PID) occurs when bacteria from chlamydia, gonorrhea, or other infections travel from the cervix into the uterus and fallopian tubes. It can cause chronic pelvic pain and infertility. The CDC classifies it alongside STIs because it’s almost always a consequence of one.
Cervicitis is inflammation of the cervix, often caused by chlamydia or gonorrhea. Urethritis is inflammation of the urethra, commonly triggered by the same infections. Epididymitis and orchitis involve inflammation of structures near or within the testicles, frequently resulting from untreated chlamydia or gonorrhea in younger men.
Testing Windows to Know
One of the most practical things to understand about STIs is that testing too soon after exposure can produce a false negative. Each infection has a “window period,” the minimum time needed before a test can reliably detect it.
Chlamydia and gonorrhea are detectable within one week in most cases, with two weeks catching nearly all infections. Syphilis and HIV require more patience: about one month catches most cases, but three months (for syphilis) or six weeks with a blood test (for HIV) provides near-complete confidence. Herpes antibody testing needs up to four months to be fully reliable. Hepatitis C has the longest window, requiring up to six months for an antibody test to catch nearly all infections.
If you’ve had a potential exposure, the timing of your test matters as much as getting the test itself. Testing during the window period and then retesting after the appropriate interval gives you the most accurate picture.