There are more than two dozen recognized sexually transmitted infections, ranging from extremely common ones like chlamydia and HPV to less familiar infections like Mycoplasma genitalium. They fall into a few broad categories based on what causes them: bacteria, viruses, or parasites. That distinction matters because it determines whether an infection can be cured or only managed.
Bacterial STIs
Bacterial infections are the most commonly reported STIs and, fortunately, the most treatable. Antibiotics can cure all of them, though the specific medication varies by infection.
Chlamydia is the single most reported STI in the United States. It often causes no symptoms at all. Roughly 77% of chlamydia cases never produce noticeable signs. When symptoms do appear, they typically include unusual discharge from the penis or vagina, burning during urination, or lower abdominal pain. Left untreated in women, chlamydia can lead to pelvic inflammatory disease and fertility problems.
Gonorrhea behaves similarly to chlamydia and often infects the same sites: the urethra, rectum, and throat. About 45% of gonorrhea cases are completely asymptomatic. Symptoms, when present, include discharge, painful urination, and in some cases sore throat or rectal discomfort depending on the site of infection. Gonorrhea and chlamydia frequently occur together.
Syphilis is unusual because it progresses through distinct stages if untreated. In the primary stage, a firm, round, painless sore appears at the infection site and lasts three to six weeks before healing on its own. The secondary stage brings a rash, often on the palms of the hands or soles of the feet, along with fever, swollen glands, sore throat, and fatigue. After that, syphilis enters a latent stage with no visible symptoms that can last years. Tertiary syphilis, which develops 10 to 30 years after the initial infection, damages internal organs including the heart, blood vessels, and brain, and can be fatal. Cases of primary and secondary syphilis declined 22% from 2023 to 2024, but congenital syphilis (passed from mother to baby during pregnancy) continues to rise, with nearly 4,000 cases reported in 2024.
Mycoplasma genitalium is a newer concern. It causes roughly 15% to 20% of urethral infections in men that aren’t caused by gonorrhea, and in women it’s linked to cervicitis, pelvic inflammatory disease, and pregnancy complications. What makes it particularly tricky is antibiotic resistance. Resistance to the most commonly used antibiotic has reached 44% to 90% in the U.S. and other Western countries, making treatment increasingly difficult.
Viral STIs
Viral STIs cannot be cured with antibiotics. Some clear on their own, others stay in the body permanently, but all can be managed.
Human papillomavirus (HPV) is the most common STI overall. Most sexually active people will contract at least one strain at some point. HPV spreads through skin-to-skin contact, not just intercourse. Low-risk strains cause genital, oral, or anal warts. High-risk strains can lead to cancers of the cervix, throat, anus, penis, vagina, and vulva. There’s no treatment for the virus itself, but there are treatments for the warts and precancerous changes it causes. Vaccination before exposure prevents infection with the most dangerous strains.
Genital herpes is caused by herpes simplex virus, usually type 2 but sometimes type 1 (the same virus behind cold sores). It causes recurring outbreaks of painful blisters or sores on the genitals, buttocks, or thighs. Many people with herpes have mild or unrecognizable symptoms and unknowingly pass the virus through skin-to-skin contact. The virus stays in the body permanently, but outbreaks tend to become less frequent over time, and daily antiviral medication can reduce both outbreaks and transmission risk.
HIV attacks the immune system and, without treatment, progresses to AIDS. Modern antiviral therapy allows most people with HIV to live long, healthy lives and reduce the virus to undetectable levels, which also prevents sexual transmission. Testing is recommended for all adults aged 13 to 64 at least once. Antibody tests can detect HIV 23 to 90 days after exposure, though newer test types can detect it sooner.
Hepatitis B and C are liver infections that can spread sexually, though they also spread through blood contact and shared needles. Hepatitis B has a vaccine. Hepatitis C does not, but it’s now curable with antiviral treatment. The CDC recommends all adults over 18 be screened for hepatitis C at least once.
Parasitic STIs
Trichomoniasis is the most common curable STI, caused by a microscopic parasite. About 70% of people with trichomoniasis have no symptoms. When symptoms appear, women typically notice itching, burning, redness, discomfort while urinating, and a thin or greenish vaginal discharge with a fishy smell. Men may notice irritation inside the penis, burning after urination or ejaculation, and discharge. Symptoms can develop 5 to 28 days after infection, or much later. It’s cured with a single course of oral medication, but both partners need treatment at the same time to prevent reinfection. Retesting about three months later is recommended even after successful treatment.
Pubic lice (sometimes called crabs) and scabies are also spread through close physical contact. Both cause intense itching and are treated with topical medications.
Why Most STIs Go Unnoticed
The defining feature of STIs as a group is how often they produce no symptoms. Chlamydia, gonorrhea, trichomoniasis, HPV, and even early syphilis can all be completely silent. This is why screening matters more than waiting for something to feel wrong.
When symptoms do show up across different STIs, they tend to overlap. Unusual discharge, painful urination, genital sores or warts, itching, and pelvic pain are the most common signs. None of these reliably points to one specific infection, which is why testing (not guessing based on symptoms) is the only way to know what you’re dealing with.
Who Should Get Tested and When
Screening recommendations vary by age, sex, and risk factors. Sexually active women under 25 should be screened for chlamydia and gonorrhea every year. Women 25 and older need annual screening only if they have risk factors like new or multiple partners. Men who have sex with men should be screened for chlamydia, gonorrhea, and syphilis at least annually, or every three to six months if at higher risk.
All pregnant women should be screened for syphilis at their first prenatal visit, with repeat testing later in pregnancy if risk factors are present. Pregnant women should also be tested for hepatitis B.
HIV screening is recommended at least once for everyone between 13 and 64, regardless of perceived risk. For cervical cancer screening (which detects the effects of HPV), women aged 21 to 29 should have a Pap test every three years. Women 30 to 65 can continue with Pap tests every three years or add HPV testing and extend the interval to every five years.
Curable vs. Manageable
A useful way to think about STIs is whether they can be eliminated from your body entirely. Bacterial infections (chlamydia, gonorrhea, syphilis, Mycoplasma genitalium) and parasitic infections (trichomoniasis) are all curable with the right medication, though antibiotic resistance is a growing challenge for gonorrhea and Mycoplasma genitalium.
Viral infections (HPV, herpes, HIV, hepatitis B) stay in the body, with one exception: hepatitis C, despite being a virus, is now curable. HPV often clears on its own within a couple of years. Herpes and HIV remain for life but are well controlled with medication. The practical difference between “curable” and “manageable” has narrowed considerably. A person on effective HIV treatment, for example, can have an undetectable viral load and a normal life expectancy.
In the U.S., more than 2.2 million cases of chlamydia, gonorrhea, and syphilis were reported in 2024. The encouraging trend is that all three of these infections declined from the prior year, with gonorrhea down 10% and syphilis down 22%. Still, the sheer volume of cases underscores how common these infections remain and why routine screening catches what symptoms alone cannot.