What Is a Sexually Transmitted Infection?

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A person can contract a sexually transmitted infection (STI) when certain bacteria, viruses, or parasites are passed from one person to another through sexual contact. The terms "sexually transmitted infection" and "sexually transmitted disease" (STD) are often used interchangeably. The difference comes down to whether a person has symptoms of infection. A person can have an STI without symptoms, but a person has an STD when an STI causes symptoms and other complications.

This article discusses the causes and types of STIs, as well as symptoms, diagnosis, and treatment, and why protection against STIs is so important.

Treatment and Prevention of STDs: Condoms, a vaccine and bottle (vaccines for certain infections such as HPV and hepatitis B), dental dams, PrEP prescription (to reduce the risk of HIV), A heart with a less than sign (limiting number of sexual partners), a medication bottle (medications for bacterial and parasitic STIs)

Verywell / Laura Porter

What Causes STIs?

Certain bacteria, viruses, and parasites cause sexually transmitted infections. The organisms can pass from one person to another in the blood, semen, saliva, or vaginal fluid.

Transmission usually occurs from unprotected vaginal, oral, or anal sex with someone who has an STI.

STI Statistics

Approximately one in five people in the United States have an STI, and almost half of new cases are among people between the ages of 15 and 24 years.


There are many types of STIs, but the most common ones include:

Half of these common STIs—the bacterial and parasitic ones—are curable. However, the viral infections (hepatitis B, herpes, HIV, and HPV) currently have no cure.

Fortunately, both HPV and hepatitis B have effective vaccines that help prevent the chances of getting an infection and, in the case of HPV, help prevent against certain cancers.


Symptoms associated with STIs vary depending on the type of infection.

Symptoms may appear a few days after exposure, but not everyone develops symptoms. Once symptoms are noticeable, the infection is classified as an STD.

General symptoms to look for include:

  • Discharge from the penis or vagina
  • Sores or warts in the genital or anal area
  • Itching or redness around the sores
  • Sores in and around the mouth
  • Painful urination
  • Abnormal vaginal odor
  • Pain during sex
  • Swollen lymph nodes in the groin

More severe infections might also have:

  • Abdominal pain
  • Fever
  • Joint pain
  • Rash to the entire body

Some people mistake symptoms of an STD for a urinary tract or yeast infection. It's important to see a healthcare provider and be tested for a wide range of STIs when symptoms begin.


STIs are diagnosed in several different ways, including:

  • Physical exam: Often a healthcare provider can diagnose an infection based on a physical or pelvic examination, particularly for infections that cause warts, rashes, or discharge.
  • Blood tests: Sometimes a blood test is used to diagnose certain infections like syphilis or HIV.
  • Urine tests: Urine testing is a standard method to test for gonorrhea and chlamydia.
  • Fluid samples: Fluid samples can be taken from the vagina or penis to be examined under a microscope for organisms like the parasite that causes trichomoniasis.

If you are sexually active, consider your need for STI testing. Local health departments are the easiest way to obtain free STI testing. Most health insurance plans and Medicaid cover the costs.

At-home STI testing is also available, and it is a convenient, user-friendly option. However, there are some considerations with at-home testing, including:

  • Insurance might not cover the costs.
  • A positive test result means that you will need to set up an appointment with a healthcare provider.
  • It's more likely that the test can give you an incorrect diagnosis when used at home.
  • Only certain states allow at-home STI testing.

If you have symptoms and suspect an STI, it's best to meet with your healthcare provider for formal testing and diagnosis.

Treatment and Prevention

Antibiotics can treat and cure STIs caused by bacteria and parasites, but they cannot cure viral infections.

For viral infections, some medications can help treat the symptoms and prevent complications. They can also help prevent the spread of the virus to another person.

Prevention is the best way to avoid an STI. Protective devices like condoms and dental dams can significantly reduce the risk of spreading an STI. There are also vaccines for certain infections, specifically HPV and hepatitis B virus.

The HPV vaccine, Gardasil 9, is recommended for everyone ages 9 to 26 years, and some people up to age 45 to help prevent cancer-causing infections and precancers. The hepatitis B vaccine is recommended for all infants and children and many groups of adults—though anyone who wants to be protected can get the vaccine.

The U.S. Food and Drug Administration (FDA) has approved the use of a combination of medicines, called HIV pre-exposure prophylaxis (PrEP), to reduce the risk of HIV infection in people at high risk. These medications require a prescription, need to be taken every day, and can be expensive.

If you have been diagnosed with an STI, your sexual partner must be tested and treated for STIs, too. Bacterial and parasitic STIs are curable with treatment, but a person can always be reinfected. Limiting your number of partners can also help prevent STIs.


When left untreated, STIs can cause significant problems, including:

  • A person not treated for gonorrhea or chlamydia can develop pelvic inflammatory disease (PID), an infection inside the uterus that can lead to problems getting pregnant. 
  • Untreated bacterial infections can also spread throughout the rest of the body and cause inflammation in the eyes, the heart, and the joints
  • HIV is a life-threatening illness that leads to AIDS and other infectious complications without treatment. 
  • Without adequate screening and treatment for HPV, people are at risk for cervical cancer and rectal cancers.

In addition, several of these infections can pass to a child during pregnancy and increase the risk of complications—including the risk of stillbirth. STI infections in newborns can lead to low birth weight, blindness, congenital deformity, and more.


Sexually transmitted infections are common, but they are easily diagnosed with various testing methods, and many of them can be treated. It's essential to see a healthcare provider if you suspect you have an STI. Prevention is the best way to avoid complications from STIs.

A Word From Verywell

Although STIs are common, they can be frustrating and embarrassing. In many cases, you may not even know that you have an STI. If you are sexually active, avoid infection by reducing your number of sexual partners and using protection. If you think you have an STI, seek testing and treatment, and make sure your partner is treated, too. Sex is more enjoyable when it's safe and free from worry about having an STI.

8 Sources
Verywell Health uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
  1. World Health Organization. Sexually transmitted infections (STIs).

  2. Centers for Disease Control and Prevention. Sexually transmitted infections prevalence, incidence, and cost estimates in the United States.

  3. US Department of Health and Human Services Office on Women's Health. Sexually transmitted infections.

  4. Lab Tests Online. At-home STD testing.

  5. MedlinePlus. Sexually transmitted diseases.

  6. Centers for Disease Control and Prevention. HPV vaccine.

  7. Centers for Disease Control and Prevention. Adult immunization schedule.

  8. US Department of Health and Human Services Office on Women's Health. Sexually transmitted infections, pregnancy, and breastfeeding.

By Christine Zink, MD
Dr. Christine Zink, MD, is a board-certified emergency medicine with expertise in the wilderness and global medicine. She completed her medical training at Weill Cornell Medical College and residency in emergency medicine at New York-Presbyterian Hospital. She utilizes 15-years of clinical experience in her medical writing.