A sexually transmitted disease (STD) is an infection passed from one person to another through sexual contact, caused by bacteria, viruses, parasites, or fungi. These infections are extremely common: in 2024 alone, the CDC recorded over 1.5 million cases of chlamydia, more than 543,000 cases of gonorrhea, and roughly 190,000 cases of syphilis in the United States. Many people carry an STI without knowing it, which is a major reason these numbers stay so high.
You’ll sometimes see the terms “STD” and “STI” used interchangeably. Technically, “STI” (sexually transmitted infection) is the broader and more current term, since many of these infections never progress to the point of causing disease. But in everyday conversation, both terms refer to the same group of conditions.
Common Types of STIs
STIs fall into a few categories based on what causes them, and that distinction matters because it determines whether the infection can be cured or only managed.
Bacterial STIs include chlamydia, gonorrhea, and syphilis. Because bacteria respond to antibiotics, these infections are generally curable, often with a single course of treatment. Bacterial vaginosis, while not always sexually transmitted, is closely linked to sexual activity and falls in this group as well.
Viral STIs include genital herpes, HPV (human papillomavirus), HIV, and hepatitis B and C. Viral infections can be treated and managed but not always cured. Antiviral medications can suppress herpes outbreaks and keep HIV in check for decades, but the virus itself remains in the body. HPV often clears on its own within a year or two, though certain strains can persist and cause health problems down the line.
Parasitic STIs include trichomoniasis, pubic lice, and scabies. Like bacterial infections, these are curable with the right medication.
How STIs Spread
Most STIs spread through vaginal, anal, or oral sex via the exchange of bodily fluids like semen, vaginal fluid, or blood. But fluid exchange isn’t the only route. Herpes and HPV spread through direct skin-to-skin contact, meaning condoms reduce the risk but don’t eliminate it entirely since they don’t cover all affected skin.
HPV in particular has a wider range of transmission than many people realize. The virus has been detected on skin and surfaces outside the context of penetrative sex, and there’s evidence it can spread through hand-to-genital contact or even self-transfer from one body part to another. Some infections, like HIV and hepatitis B, can also pass from a mother to her baby during childbirth or through shared needles.
Why Many Infections Go Unnoticed
One of the most important things to understand about STIs is that the majority produce no obvious symptoms, especially early on. With chlamydia, only about 11 to 33 percent of infected men and 6 to 17 percent of infected women ever develop noticeable symptoms. Gonorrhea is somewhat more likely to cause symptoms in men (45 to 85 percent of cases), but only 14 to 35 percent of women with gonorrhea will notice anything wrong.
Herpes follows a similar pattern. In one study tracking people who newly acquired HSV-2 (the virus most associated with genital herpes), only 37 percent developed symptoms during the observation period. The rest had no idea they were infected. This is why routine screening matters so much. Waiting for symptoms to appear before getting tested means many infections go undiagnosed and continue to spread.
Testing and Window Periods
Getting tested too soon after exposure can produce a false negative, because it takes time for an infection to become detectable. Each STI has a “window period,” the gap between exposure and when a test can reliably pick it up.
- Chlamydia and gonorrhea: Typically detectable within one to two weeks after exposure.
- Syphilis: A blood test catches most infections at one month, and nearly all by three months.
- HIV: A blood test using antigen/antibody methods catches most cases at two weeks and almost all by six weeks. Oral swab tests take longer, with most detectable at one month and nearly all by three months.
- Hepatitis B: Detectable by blood test at three to six weeks after exposure.
- Hepatitis C: Most infections show up at two months, though it can take up to six months for a definitive result.
If you’ve had a specific exposure you’re concerned about, testing once at the early end of the window and again at the later end gives you the most reliable answer.
What Happens Without Treatment
Untreated STIs don’t just linger. They can cause serious, sometimes irreversible damage. The most well-documented example is pelvic inflammatory disease (PID), which develops when infections like chlamydia or gonorrhea spread to the uterus, fallopian tubes, or ovaries.
PID carries significant long-term consequences. In one major clinical trial, 18 percent of participants with PID experienced infertility within three years. Women with a history of PID face more than double the risk of ectopic pregnancy (a dangerous condition where a fertilized egg implants outside the uterus). Chronic pelvic pain develops in 18 to 36 percent of PID cases and can climb as high as 67 percent in women who have had three or more episodes. Each recurrence compounds the damage, because prior tubal scarring fuels a cycle of ongoing inflammation.
Other untreated STIs carry their own risks. Syphilis progresses through stages and can eventually damage the brain, heart, and other organs. Certain HPV strains cause cervical, throat, and anal cancers. Untreated HIV gradually destroys the immune system, leaving the body unable to fight off infections that would otherwise be harmless.
Prevention
Condoms remain one of the most effective tools for reducing STI transmission during vaginal, anal, and oral sex. They’re especially effective against infections spread through fluids (like HIV, gonorrhea, and chlamydia) and somewhat less protective against skin-to-skin infections like herpes and HPV, though they still lower the risk.
Vaccination offers strong protection against two of the most consequential viral STIs. The HPV vaccine is recommended starting at age 11 or 12, though it can be given as early as age 9. For those who missed it, catch-up vaccination is recommended through age 26. Children who start the series between ages 9 and 14 need only two doses spaced six to twelve months apart. Those who start at 15 or older need three doses spread over six months. A hepatitis B vaccine is also widely available and part of the standard childhood immunization schedule.
Regular screening is the other critical piece, particularly because so many infections are silent. How often you should be tested depends on your age, sexual activity, and number of partners, but annual screening for chlamydia and gonorrhea is standard guidance for sexually active women under 25 and for anyone with new or multiple partners.
Curable vs. Lifelong Infections
It helps to think of STIs in two buckets. Bacterial and parasitic infections (chlamydia, gonorrhea, syphilis, trichomoniasis) are curable, often with a single dose or short course of antibiotics. Once treated, the infection is gone, though you can absolutely catch it again through future exposure.
Viral infections (herpes, HPV, HIV, hepatitis B) are a different story. Treatment can suppress symptoms and, in the case of HIV, reduce the virus to undetectable levels where it can no longer be transmitted sexually. But the virus itself typically persists in the body. Living with a viral STI is very manageable with modern medicine, but it does mean ongoing treatment or monitoring rather than a one-time cure.