Sexually transmitted infections fall into four categories based on what causes them: bacteria, viruses, parasites, and fungi. Each type behaves differently in the body, produces different symptoms, and requires a different treatment approach. Some are curable with a short course of antibiotics, while others stay in the body permanently but can be managed. Globally, over 1 million new cases of just the four most common curable STIs occur every day.
Bacterial STIs
Bacterial STIs are caused by bacteria and are generally curable with antibiotics. The three most well-known are chlamydia, gonorrhea, and syphilis.
Chlamydia is one of the most frequently reported STIs, partly because it so often flies under the radar. Up to 90% of women with chlamydia have no symptoms at all, which is why annual screening is recommended for sexually active women under 25. When symptoms do appear, they typically include unusual discharge or a burning sensation during urination. Left untreated, chlamydia can lead to pelvic inflammatory disease, which can cause chronic pain and fertility problems. The good news: a standard course of antibiotics clears it up.
Gonorrhea can infect the genitals, rectum, and throat. Symptoms resemble chlamydia (discharge, burning, sometimes none at all), and the two infections frequently occur together. What sets gonorrhea apart is a growing resistance to antibiotics. Over the past two decades, the bacteria has developed resistance to one class of drugs after another. Currently, only one class of antibiotics remains effective in the U.S. While resistance to that class has stayed low (under 0.1% of tested samples in 2019), public health officials watch it closely.
Syphilis progresses through distinct stages if untreated. It starts with a painless sore (called a chancre) at the site of infection, then moves to a body-wide rash, and can eventually cause serious damage to the heart, brain, and other organs years later. Syphilis is fully curable in its early stages. The World Health Organization estimated 8 million new syphilis infections among adults in 2022, along with 700,000 cases of congenital syphilis passed from mother to baby during pregnancy.
Two lesser-known bacterial STIs worth mentioning are mycoplasma genitalium (Mgen) and bacterial vaginosis (BV). Mgen can cause urethritis and pelvic inflammation, and BV disrupts the normal balance of vaginal bacteria. Both are treatable with antibiotics.
Viral STIs
Viral STIs are caused by viruses and, with few exceptions, cannot be cured. They can be managed, sometimes very effectively, but the virus typically remains in the body.
Human papillomavirus (HPV) is the most common STI overall. Most sexually active people will contract at least one strain of HPV in their lifetime. Many strains cause no problems and clear on their own, but certain high-risk strains can lead to cervical, throat, and anal cancers. Other strains cause genital warts. HPV spreads through skin-to-skin contact, meaning condoms reduce but don’t eliminate the risk. Vaccination before exposure is highly effective at preventing the most dangerous strains.
Genital herpes (HSV) is caused by herpes simplex virus, usually type 2 but sometimes type 1 (the same virus behind cold sores). Like HPV, herpes spreads through skin-to-skin contact, not just fluid exchange. Most people with herpes don’t know they have it because outbreaks can be mild or absent entirely. When symptoms appear, they involve painful blisters or sores around the genitals or mouth. Antiviral medications reduce the frequency and severity of outbreaks and lower the chance of passing the virus to a partner, but they don’t eliminate the virus from the body.
HIV attacks the immune system, specifically the cells your body relies on to fight off infections. Without treatment, HIV progresses to AIDS over several years. With modern antiretroviral therapy, people with HIV can maintain an undetectable level of virus in their blood, live a normal lifespan, and cannot transmit the virus to sexual partners. That transformation from fatal diagnosis to manageable condition is one of the major success stories of modern medicine, but it depends on early detection and consistent treatment.
Hepatitis B and C both affect the liver and can be transmitted sexually, though hepatitis C spreads more commonly through blood contact. Hepatitis B has no cure but is preventable with a vaccine. Hepatitis C can now be cured in most cases with antiviral treatment lasting 8 to 12 weeks.
Parasitic STIs
Trichomoniasis is the most common curable STI worldwide. It’s caused by a microscopic parasite and is especially common in women, often producing a frothy, foul-smelling vaginal discharge, itching, and discomfort during urination or sex. Many cases, however, produce no symptoms. A single dose of antiparasitic medication cures it.
Pubic lice (commonly called crabs) are tiny insects that infest pubic hair and cause intense itching. They spread through close body contact and are visible to the naked eye as small, crab-shaped creatures clinging to hair shafts. Scabies is caused by a microscopic mite that burrows into the skin, producing an itchy rash. The first time you’re infested, the itching may take weeks to develop as your body becomes sensitized to the mites. A second infestation can cause symptoms within 24 hours. Both conditions are treatable with topical or oral medications.
How STIs Spread Differently
Not all STIs spread the same way, and understanding the differences matters for protection. Most bacterial STIs and HIV transmit through bodily fluids during vaginal, oral, or anal sex. Herpes and HPV spread through direct skin-to-skin contact, which means areas not covered by a condom can still transmit the virus. Pubic lice and scabies spread through close physical contact that doesn’t necessarily involve intercourse.
This is why condoms significantly reduce but don’t completely eliminate the risk for every STI. For HPV and hepatitis B, vaccines offer a layer of protection that condoms can’t fully provide.
Many STIs Have No Symptoms
One of the most important things to understand about STIs is that many produce no obvious signs. Chlamydia is asymptomatic in the vast majority of women. Most people with herpes are unaware they carry it. HIV can take years to produce noticeable illness. This is why regular screening matters more than waiting for symptoms to appear.
Testing windows vary by infection. Chlamydia and gonorrhea can be reliably detected about two weeks after exposure using a urine sample or swab. Syphilis takes longer: a blood test catches most cases at one month, but three months covers nearly all. HIV detection depends on the test type. A blood test using the newer antigen/antibody method picks up most infections at two weeks, with six weeks catching almost all cases. Older oral swab tests require up to three months. Herpes antibody testing needs one to four months to become reliable, and hepatitis C can take up to six months.
Curable vs. Manageable
A useful way to think about STIs is whether they can be eliminated from your body or only controlled. Bacterial STIs (chlamydia, gonorrhea, syphilis) and parasitic infections (trichomoniasis, pubic lice, scabies) are all curable, typically with a short treatment. Hepatitis C, though viral, is also curable with newer antiviral therapies.
Herpes, HPV, HIV, and hepatitis B are not curable but are manageable. HPV often clears on its own within a year or two. Herpes outbreaks can be suppressed with daily medication. HIV can be controlled to undetectable levels. Hepatitis B is monitored and, in some cases, treated with antivirals to prevent liver damage. Living with any of these conditions looks very different today than it did even 20 years ago.