Sexually transmitted diseases (STDs) are caused by infections passed through vaginal, anal, or oral sex. The World Health Organization estimated 374 million new infections in 2020 from just four of them: chlamydia, gonorrhea, syphilis, and trichomoniasis. Beyond those, viral infections like herpes, HPV, and HIV affect hundreds of millions more. Here’s what each one looks like, how it spreads, and what to know about treatment.
A quick note on terminology: “STI” refers to the infection itself, while “STD” refers to the disease that develops when that infection causes symptoms. Many sexually transmitted infections never produce noticeable symptoms, which is why the medical world has shifted toward using “STI.” Both terms describe the same group of conditions.
Chlamydia
Chlamydia is the most commonly reported bacterial STI, with an estimated 129 million new cases worldwide in 2020. It’s caused by bacteria and spreads through vaginal, anal, or oral sex. The tricky part is that asymptomatic infection is common in both men and women, meaning many people carry and spread it without ever knowing.
When symptoms do appear, they typically include pain during urination, unusual discharge from the penis or vagina, and in women, bleeding between periods or during sex. Left untreated, chlamydia can cause pelvic inflammatory disease, which can lead to chronic pain and fertility problems. In men, it can cause painful swelling in the testicles. A short course of oral antibiotics clears the infection, and testing is straightforward with a urine sample or swab.
Gonorrhea
Gonorrhea accounted for roughly 82 million new infections globally in 2020. Like chlamydia, it often produces no symptoms, especially in women. When symptoms do show up, they include painful urination, discharge, and in some cases rectal pain or bleeding if the infection was transmitted through anal sex.
What makes gonorrhea particularly concerning is its growing resistance to antibiotics. Over the past two decades, the bacteria have become resistant to fluoroquinolones, penicillin, tetracycline, and several other drug classes that once worked well. Only one antibiotic remains recommended as a first-line treatment in the United States. Untreated gonorrhea can cause the same fertility complications as chlamydia and can also spread to the blood and joints, causing a potentially serious systemic infection.
Syphilis
Syphilis progresses through distinct stages, each with different symptoms and risks. An estimated 7 to 8 million adults were infected globally in recent years, and case counts have been climbing in many countries.
Primary Stage
The first sign is usually a single sore (or sometimes multiple sores) at the site where the bacteria entered the body. These sores are typically firm, round, and painless, which means they’re easy to miss, especially if they’re inside the mouth, vagina, or rectum. The sore lasts 3 to 6 weeks and heals on its own whether or not you get treated. But healing doesn’t mean the infection is gone.
Secondary Stage
Weeks after the initial sore heals, a rash may appear on one or more areas of the body, including the palms of the hands and soles of the feet. It often looks rough and reddish-brown. Other symptoms at this stage include fever, swollen lymph nodes, sore throat, patchy hair loss, headaches, weight loss, muscle aches, and fatigue.
Latent and Tertiary Stages
If still untreated, syphilis enters a latent stage with no visible symptoms. This can last years. In some people, the infection eventually progresses to tertiary syphilis, which typically appears 10 to 30 years after the initial infection. At this point, the disease can damage the heart, blood vessels, brain, and nervous system, and it can be fatal. Syphilis is fully curable with antibiotics at any stage, but damage done during the tertiary stage cannot be reversed.
HPV (Human Papillomavirus)
HPV is the most common sexually transmitted infection overall. Most sexually active people will contract at least one strain of HPV at some point. The majority of infections clear on their own within a year or two without causing problems, but certain strains carry serious cancer risks.
There are 12 high-risk HPV types. These strains are responsible for virtually all cervical cancers and about 70% of oropharyngeal (throat) cancers. They also contribute to cancers of the anus, penis, vagina, and vulva. Low-risk strains, by contrast, cause genital warts but not cancer.
The HPV vaccine is highly effective at preventing infection with the most dangerous strains and is recommended for preteens but can be given through age 26, and in some cases up to 45. Routine cervical screening (Pap tests and HPV tests) catches precancerous changes early, which is why regular screening matters even if you’ve been vaccinated.
Genital Herpes (HSV-1 and HSV-2)
Herpes is extraordinarily common. An estimated 3.8 billion people under age 50 (about 64% of the global population) have HSV-1, while roughly 520 million people aged 15 to 49 have HSV-2. HSV-1 traditionally causes oral cold sores but increasingly causes genital herpes through oral sex. HSV-2 is the primary cause of genital herpes.
Most herpes infections are asymptomatic or unrecognized. Many people pass the virus to partners without knowing they carry it, because both HSV-1 and HSV-2 can be transmitted even when no sores are visible and no symptoms are present. When outbreaks do occur, they involve painful blisters or sores around the genitals, rectum, or mouth. The first outbreak is usually the most severe, with subsequent ones tending to be milder and shorter.
There is no cure for herpes. Antiviral medications reduce the frequency and severity of outbreaks and lower the risk of transmission, but the virus stays in the body permanently. For most people, herpes is more of a recurring nuisance than a serious health threat, though it can cause complications during pregnancy and for people with weakened immune systems.
HIV
HIV attacks the immune system, specifically the cells your body needs to fight off infections. Without treatment, it progresses to AIDS, a condition where the immune system is too damaged to protect against serious illnesses and certain cancers.
Early HIV infection often produces flu-like symptoms (fever, sore throat, rash, fatigue) within 2 to 4 weeks of exposure, but some people have no symptoms for years. The only way to know your status is through testing.
Modern treatment has transformed HIV from a fatal diagnosis into a manageable chronic condition. People with HIV who take antiretroviral therapy and maintain a very low viral load (below 200 copies per milliliter of blood) cannot transmit the virus sexually. This is known as Undetectable = Untransmittable, or U=U. Prevention has also advanced significantly: pre-exposure prophylaxis (PrEP) reduces the risk of getting HIV from sex by about 99% when taken as prescribed. An injectable form of PrEP, given every two months, is over 99% effective as well.
Trichomoniasis
Trichomoniasis is caused by a parasite rather than bacteria or a virus, making it the most common curable non-viral STI. An estimated 156 million new cases occurred worldwide in 2020.
Between 70% and 85% of infected people have minimal or no symptoms, and untreated infections can last months to years. When symptoms do appear in women, they may include a yellow-green, foul-smelling vaginal discharge and vulvar irritation. Men may experience discomfort during urination or ejaculation. A single oral dose of an anti-parasitic medication cures the infection in most cases.
Hepatitis B
Hepatitis B is a viral infection that attacks the liver. While it spreads through blood and other body fluids (not exclusively through sex), sexual contact is a major route of transmission. An estimated 254 million people were living with chronic hepatitis B in 2022.
Many adults who contract hepatitis B clear the virus on their own within six months. But some develop a chronic infection that can silently damage the liver over decades, potentially leading to cirrhosis or liver cancer. A highly effective vaccine exists and is part of routine childhood immunization in most countries. If you weren’t vaccinated as a child, getting the series as an adult provides strong, long-lasting protection.
When and How to Get Tested
Because so many STIs produce no symptoms, testing is the only reliable way to know your status. How soon a test can detect an infection after exposure varies by the specific STI:
- HIV (blood test): A newer antigen/antibody blood test catches most infections within 2 weeks, and nearly all by 6 weeks. An oral swab test takes longer, catching most by 1 month and nearly all by 3 months.
- Syphilis: Blood tests catch most infections within 1 month, and nearly all by 3 months.
- Hepatitis B: Blood tests are typically accurate at 3 to 6 weeks after exposure.
- Chlamydia and gonorrhea: Urine or swab tests are generally reliable about 2 weeks after exposure.
Testing too early can produce a false negative, so timing matters. If you’ve had a recent exposure and your first test comes back negative, retesting after the full window period gives you a more reliable result. Many clinics and community health centers offer free or low-cost STI testing, and home test kits are available for several infections including HIV, chlamydia, and gonorrhea.