Sexually transmitted diseases (STDs), also called sexually transmitted infections (STIs), fall into three main categories: bacterial, viral, and parasitic. In the United States alone, provisional 2024 data from the CDC recorded over 1.5 million chlamydia cases, more than 543,000 gonorrhea cases, and over 190,000 syphilis cases. Here’s what each type looks like, how they differ, and what you should know about each one.
Why Categories Matter
The type of organism causing an STI determines almost everything about it: whether it can be cured, how it’s treated, how long it takes to show up on a test, and what happens if it goes untreated. Bacterial STIs are generally curable with antibiotics. Viral STIs are typically manageable but not curable. Parasitic STIs are curable, often with a single course of treatment. Knowing which category an infection falls into helps you understand what to expect.
Bacterial STIs
Chlamydia
Chlamydia is the most commonly reported bacterial STI in the U.S. It’s caused by the bacterium Chlamydia trachomatis and can infect the genitals, rectum, and throat. The tricky part: most infections produce no symptoms at all. Studies estimate that roughly 61% of chlamydia infections in women are completely asymptomatic.
When symptoms do appear, they can include painful urination, unusual discharge, bleeding between periods, or testicular pain. Left untreated, chlamydia can lead to pelvic inflammatory disease in women, which can cause chronic pain and infertility. It’s treated with a week-long course of antibiotics.
Gonorrhea
Gonorrhea is caused by the bacterium Neisseria gonorrhoeae and infects the same sites as chlamydia. It’s also frequently silent, with an estimated 53% of infections in women causing no symptoms. When symptoms do show up, they overlap heavily with chlamydia: discharge, painful urination, sore throat, or rectal discomfort. Because the two infections look so similar and often occur together, testing for both at the same time is standard practice.
Gonorrhea is treated with an injectable antibiotic, and antibiotic resistance has become a growing concern. If chlamydia hasn’t been ruled out, an additional oral antibiotic is prescribed alongside.
Syphilis
Syphilis, caused by the bacterium Treponema pallidum, is unique because it progresses through distinct stages if untreated. In the primary stage, a firm, round, painless sore appears at the site of infection, usually on the genitals, anus, or mouth. Because it doesn’t hurt, many people never notice it. The sore heals on its own within three to six weeks, but the infection doesn’t go away.
The secondary stage brings a rash that can appear on the palms of the hands and soles of the feet, along with fever, swollen lymph nodes, sore throat, patchy hair loss, and fatigue. After this, syphilis enters a latent stage with no visible symptoms that can last years. In rare cases, untreated syphilis reaches a tertiary stage 10 to 30 years later, damaging the heart, blood vessels, brain, and nervous system. At that point, it can be fatal. Syphilis is curable with antibiotics at any stage, though damage from late-stage disease may be permanent.
Mycoplasma Genitalium
This lesser-known bacterial STI is increasingly on clinicians’ radar. Mycoplasma genitalium is responsible for roughly 15 to 20% of cases of urethritis in men that aren’t caused by gonorrhea. In women, it’s linked to cervicitis, pelvic inflammatory disease, preterm delivery, and infertility, with about a twofold increase in risk for these outcomes.
The major challenge with this infection is antibiotic resistance. Resistance to common antibiotics has been confirmed at alarming rates, with molecular markers for resistance found in 44 to 90% of samples tested in the U.S., Canada, Western Europe, and Australia. Standard single-dose treatments that used to work now frequently fail and can even drive further resistance. Treatment now requires resistance-guided testing to pick an antibiotic that will actually work.
Viral STIs
Herpes (HSV-1 and HSV-2)
Two types of herpes simplex virus cause infections. HSV-1 mostly spreads through oral contact and typically causes cold sores around the mouth, though it can also cause genital herpes. HSV-2 spreads through sexual contact and primarily causes genital herpes. HSV-2 is much more likely than genital HSV-1 to cause recurring outbreaks.
After a first outbreak, subsequent episodes tend to be shorter and less severe. Many people with herpes have mild or no symptoms and don’t realize they carry the virus. There’s no cure, but antiviral medications reduce the frequency and severity of outbreaks and lower the risk of passing the virus to partners.
Human Papillomavirus (HPV)
HPV is the most common STI in the United States. There are many different strains, and most clear on their own without causing problems. Some low-risk strains cause genital warts. High-risk strains, particularly types 16 and 18, can cause cancers of the cervix, throat, anus, and genitals.
The HPV vaccine (Gardasil-9) protects against nine strains, including the two responsible for most HPV-related cancers and two that cause genital warts. The vaccine has the potential to prevent more than 90% of HPV-related cancers. Since vaccination programs began, infections with cancer-causing HPV types have dropped 88% among teen girls and 81% among young adult women. Cervical pre-cancers linked to those same HPV types have fallen by 40% among vaccinated women.
HIV
Human immunodeficiency virus attacks the immune system, and without treatment it progresses to AIDS. HIV spreads through vaginal or anal sex, sharing injection equipment, and from mother to child during pregnancy or breastfeeding. Having another STI, particularly syphilis or gonorrhea, increases the risk of acquiring HIV.
Modern treatment keeps the virus suppressed to undetectable levels, which means a person on effective treatment cannot transmit HIV sexually. For prevention, pre-exposure prophylaxis (PrEP) is available as a daily pill or a long-acting injection for people at increased risk. PrEP is highly effective when taken as directed. A blood test using antigen/antibody detection picks up most HIV infections within two weeks, with nearly all infections detectable by six weeks.
Hepatitis B
Hepatitis B is an inflammatory liver disease caused by a virus that spreads through sexual contact, blood, and from mother to child. Most adults clear the infection on their own, but some develop a chronic infection that 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 can detect the infection roughly three to six weeks after exposure.
Parasitic STIs
Trichomoniasis
Trichomoniasis is caused by the parasite Trichomonas vaginalis and is one of the most common curable STIs. About 57% of infections in women produce no symptoms. When symptoms do appear, they typically include irritation, itching, unusual discharge with a strong odor, or discomfort during urination or sex. It’s treated with oral antibiotics, and both partners need treatment at the same time to prevent passing it back and forth.
Pubic Lice and Scabies
Pubic lice (sometimes called crabs) and scabies are ectoparasites, meaning they live on or in the skin rather than inside the body. Pubic lice cause intense itching in the pubic area, and the lice or their eggs can be seen on the hair. They’re treated with a medicated cream rinse applied for 10 minutes.
Scabies is caused by tiny mites that burrow into the skin, producing an itchy rash that’s often worst at night. It can spread through prolonged skin-to-skin contact, including sexual contact. Treatment involves a prescription cream applied from the neck down and left on overnight, or an oral medication. Bedding and clothing should be washed in hot water at the same time to prevent reinfestation.
Most STIs Are Silent
The single most important thing to understand about STIs is that the majority don’t announce themselves. Across chlamydia, gonorrhea, and trichomoniasis, roughly half to two-thirds of infections produce zero symptoms. Syphilis sores are painless. Many people with herpes or HPV never know they’re infected. This is exactly why routine screening matters: waiting for symptoms means missing the majority of infections.
Testing Windows to Know
Getting tested too early after a potential exposure can produce a false negative. Each infection has a window period, the minimum time needed for a test to reliably detect it. For HIV, a blood-based antigen/antibody test catches most infections by two weeks and nearly all by six weeks. An oral swab for HIV takes longer, catching most by one month and nearly all by three. Syphilis blood tests detect most cases at one month, with three months needed to catch nearly all infections. Hepatitis B shows up on blood tests at roughly three to six weeks.
Chlamydia and gonorrhea can typically be detected by urine or swab test within one to two weeks of exposure. If you’re concerned about a specific encounter, knowing these windows helps you time your testing so the results are meaningful.