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Sexually Transmitted Infections
Reviewed by Medicinsk Expert•Last updated: 2026-10-22•Women

Sexually Transmitted Infections

Sexually transmitted infections (STIs) are passed through sexual contact and, if left untreated, can lead to serious health complications or develop into sexually transmitted diseases (STDs).

What are sexually transmitted infections (STIs)?

Sexually transmitted infections (STIs) are infections that are passed from one person to another through sexual contact. If not treated, an STI can develop into a sexually transmitted disease (STD).

STIs are usually spread during vaginal, oral, or anal sex. But sometimes they can spread through other sexual contact involving the penis, vagina, mouth, or anus. This is because some STIs, like herpes and HPV, are spread by skin-to-skin contact.

Some STIs can be passed to the fetus during pregnancy or childbirth. Other ways that some STIs may spread include while breastfeeding, through blood transfusions, or by sharing needles.

There are more than 20 types of STIs, including:

  • Chlamydia
  • Genital herpes
  • Genital warts
  • Gonorrhea
  • HIV
  • HPV
  • Pubic lice
  • Syphilis
  • Trichomoniasis

What causes sexually transmitted infections (STIs)?

STIs can be caused by bacteria, viruses, and parasites.

Who is affected by sexually transmitted infections (STIs)?

Most STIs affect both men and women, but in many cases the health problems they cause can be more severe for women. If an STI is passed to a fetus during pregnancy or childbirth, it can cause serious health problems.

What are the symptoms of sexually transmitted infections (STIs)?

STIs don't always cause symptoms or may only cause mild symptoms. So, it is possible to have an infection and not know it. And even without symptoms, STIs can still be harmful and may be passed on during sex.

If you do have symptoms, they could include:

  • Unusual discharge from the penis or vagina
  • Sores or warts on the genital area
  • Painful or frequent urination (peeing)
  • Itching and redness in the genital area
  • Blisters or sores in or around the mouth
  • Abnormal vaginal odor
  • Anal itching, soreness, or bleeding
  • Abdominal (belly) pain
  • Fever

How are sexually transmitted infections (STIs) diagnosed?

If you are sexually active, you should talk to your health care provider about your risk for STIs and whether you need to be tested. This is especially important since many STIs do not usually cause symptoms.

Some STIs may be diagnosed during a physical exam or through STI testing. Some tests involve a microscopic examination of a sore or fluid swabbed from the vagina, penis, or anus. Blood tests can diagnose other types of STIs.

What are the treatments for sexually transmitted infections (STIs)?

Antibiotics can treat STIs that are caused by bacteria or parasites. There is no cure for STIs caused by viruses, but antiviral medicines can often help with the symptoms and lower your risk of spreading the infection.

It's important to get treatment if you have an STI. Some types of STIs can cause serious health problems if they are not treated. Talk to your provider about how long to not have sex after treatment to avoid passing the infection back and forth with your partner.

Can sexually transmitted infections (STIs) be prevented?

The surest way to protect yourself against STIs is to not have sex.

If you do decide to have sex, you can lower your risk of getting an STI by:

  • Having you and your partner tested for STIs before having sex.
  • Using a condom every time you have sex. Correct usage of latex condoms greatly reduces, but does not eliminate, the risk of catching or spreading STIs. If you or your partner is allergic to latex, you can use polyurethane condoms.
  • Reducing your number of sex partners. You might even consider "mutual monogamy" with just one partner. This means that you agree to be sexually active only with them, and they agree to be sexually active only with you. But first you should both get tested to make sure that you don't already have an STI.
  • Getting vaccinated against HPV and hepatitis B.

Frequently Asked Questions

Yes, many STIs—including herpes, HPV, gonorrhea, and chlamydia—can be transmitted via oral sex. Use barrier methods such as condoms or dental dams during oral sex, avoid contact with active sores, and maintain good oral hygiene. Regular screening for oral infections is also recommended for sexually active individuals.
Untreated chlamydia can lead to pelvic inflammatory disease (PID) in women, which may cause chronic pelvic pain, ectopic pregnancy, and infertility. In men, it can cause epididymitis and infertility. Untreated gonorrhea can also cause PID, infertility, and increased risk of HIV transmission. Both infections can spread to the bloodstream and cause serious complications.
Certain STIs (e.g., syphilis, HIV, herpes, chlamydia, gonorrhea) can be transmitted to the fetus, leading to congenital infections, miscarriage, stillbirth, or neonatal complications. Pregnant women should be screened early in pregnancy, especially if they have risk factors. If an STI is detected, prompt treatment with appropriate antibiotics or antiviral therapy is essential to protect both mother and baby.===
Regular screening is the most reliable method. For sexually active adults, CDC recommends annual testing for chlamydia and gonorrhea for all women under 25 and for men who have sex with men. HIV screening should be done at least once for everyone, and more often if at higher risk. Use a reputable clinic or your primary care provider for confidential testing.
Source: MedlinePlus, National Library of Medicine

MedlinePlus brings together authoritative health information from the National Library of Medicine (NLM), the National Institutes of Health (NIH), and other government agencies and health-related organizations.

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