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Sexually Transmitted Infections (STIs)

  • Infectious diseases that spread from person to person through vaginal, anal, or oral sex
  • Some of the types are HIV, HPV, chlamydia, gonorrhea, syphilis, genital herpes, trichomoniasis
  • Treatment depends on the cause of infection, and can include antibiotics or antiviral drugs
  • Involves infectious diseases, pediatric infectious diseases, AIDS care program, urology, obstetrics, gynecology & reproductive sciences

Sexually Transmitted Infections (STIs)


Sexually transmitted diseases (STIs), also known as sexually transmitted diseases (STDs), might be difficult to talk about, but it’s important to realize the risks they pose and how common they are. According to the Centers for Disease Control and Prevention (CDC), between 2015 and 2019 the reported cases of chlamydia, gonorrhea, and syphilis increased by almost 30%. The health agency estimates that in 2018, one in five people in the United States had an STI.

While STIs can affect people of any age, they are most commonly reported in adolescents and young adults. In 2020, for example, people between the ages of 15 and 24 accounted for 53% of reported STI cases.

“What’s often not understood is that many of these STIs can have long-term consequences, especially for women,” says Sangini S. Sheth, MD, MPH, a Yale Medicine obstetrician-gynecologist. Having had an STI puts a woman at higher risk for a wide range of conditions, including pelvic pain, fertility problems, and increased risk of ectopic pregnancies (when the fertilized egg implants outside the uterus). For men, STIs raise the risk for lifelong infections, infertility, and possibly cancer.

Since many STIs have no symptoms, people can unknowingly spread infections to their sexual partners. Fortunately, many STIs can be cured and for those that cannot be cured yet, medications are available that can effectively lessen symptoms and reduce the risk of transmission.

What are sexually transmitted infections (STIs)?

STIs are infectious diseases caused by bacteria, viruses, and parasites that spread from person to person through vaginal, anal, and oral sex. Some STIs can also spread from other activities such as deep kissing. A mother can also transmit an STI to her child during pregnancy.

Common STIs include:

  • HIV: Human immunodeficiency virus, or HIV, attacks the immune system, the natural defense against illness. HIV causes AIDS (acquired immunodeficiency syndrome). In the first two to four weeks after HIV infection, most people experience flu-like symptoms that usually last for a few days to a couple of weeks. Following this initial period of infection, many people do not have any further symptoms for several years. Without treatment, the virus damages the immune system. AIDS is diagnosed when the immune system has been severely damaged, usually resulting in opportunistic infections.
  • HPV: Human papillomavirus (HPV) is a group of more than 150 related viruses. Each gets its own number, called its HPV type. Many forms of HPV cause warts (papillomas) that are often found on the inside or outside of the genitals. Genital warts can spread to surrounding skin or to a sexual partner. Some HPV types have no symptoms and disappear without ever causing health problems. However, infection by some types can lead to abnormal changes in the cells, which can progress to cervical cancer as well as cancer of the vagina, vulva, penis, anus, or throat.
  • Chlamydia: This STI is caused by bacteria called Chlamydia trachomatis. Women get the infection in their cervix, urethra, or rectum. Men can get it in their urethra or rectum. In both men and women, it can cause conjunctivitis, an eye infection. Symptoms may include abnormal genital discharge and burning during urination. In women, chlamydia can lead to pelvic inflammatory disease (an infection in the uterus, ovaries, or fallopian tubes), tubal (ectopic) pregnancies, and infertility. If passed on to a baby, it can cause eye infections or pneumonia. Chlamydia testing is important because it’s treatable with antibiotics. Because it doesn’t always bring symptoms, many people with chlamydia don’t know they have it.
  • Gonorrhea: Caused by the bacteria Neisseria gonorrhoeae, gonorrhea can infect the mucous membranes of the urethra, vagina, penis, anus, mouth, throat, and/or eyes. Symptoms include burning and pain during urination, a discharge from the penis or vagina, and frequent urination, among others. If the infection spreads to the uterus or fallopian tubes, it can cause pelvic inflammatory disease, infertility, and ectopic pregnancy. In rare cases, the infection can spread to the blood, causing a condition called disseminated gonococcal infection (DGI) that is marked by arthritis and joint pain, fever and chills, and dermatitis. Gonorrhea can also be spread during childbirth and cause serious eye infections in babies.
  • Genital herpes: This condition is caused by the herpes simplex virus (HSV) and usually appears as one or more blisters on or around the genitals, rectum, or mouth. When the blisters pop, they leave painful sores, which may take weeks to heal. There is no cure for HSV. The virus stays in the body, and the sores may return. Medicine can shorten an outbreak or help with symptoms. The virus is spread during sexual intercourse or from the mouth to the genitals during oral sex, even when a person has no visible blisters. It can also be spread to babies during childbirth.
  • Syphilis: This STI, caused by the bacteria Treponema pallidum, can be described in stages. The first stage occurs a week to three months after infection. In this stage, a sore—or sometimes multiple sores—forms on the skin where the infection occurred, typically on the penis, vulva, vagina, anus, rectum, or mouth. The sore usually resolves within three to six weeks.

    During the second stage, the infection spreads around the body via the blood. Symptoms commonly include a skin rash that may look wart-like in most parts of the body (armpits, genitals, anus, to name a few) and can include patchy hair loss, lymph node swelling, and potentially liver inflammation or visual/hearing symptoms.

    The infection can then become silent for years—some individuals never have a problem for the rest of their lives, but up to 20-30% can go on to develop late complications, known as tertiary syphilis, that may affect the brain, eyes, nerves, heart, blood vessels, bones, liver, or joints, and can result in death. Syphilis is treated with antibiotics.
  • Trichomoniasis: Also known as “trich,” this is an infection caused by a parasite called Trichomonas vaginalis. Symptoms can vary (but usually take the form of genital irritation) and only arise in about 30% of people who have it. In women, it most commonly causes vaginal discharge. In men, it most commonly affects the urethra.

What are the symptoms of STIs?

STIs can have a wide array of signs and symptoms—and sometimes none at all. As a result, an infection might progress unnoticed until serious problems develop and/or a sexual partner is infected and diagnosed with an STI. Depending on the STI, symptoms can arise within days of exposure or years later.

Signs and symptoms that might indicate an STI include:

  • Sores or bumps in, on or around the genitals or in the mouth or rectal area
  • Rash, especially over the torso, hands, or feet
  • Burning or painful urination
  • Unusual vaginal bleeding
  • Discharge from the penis
  • Unusual or strange smelling vaginal discharge
  • Pain during sex
  • Lower abdominal pain
  • Fever

STIs can cause long-term harm, including chronic pain and infertility. Each year, untreated STIs cause infertility in at least 24,000 women in the U.S., the Centers for Disease Control and Prevention (CDC) reports. Untreated syphilis in pregnant women causes infant death in up to 40% of cases.

What are the risk factors for STIs?

Several behavioral factors can increase the risk of getting an STI, including:

  • Having vaginal, anal, or oral sex without a condom outside of a monogamous relationship
  • Improper or inconsistent use of condoms during sex
  • Having multiple sex partners or having sex with someone who has multiple sex partners
  • Having sex with sex workers
  • Having sex with someone who has been treated recently for an STI
  • Sharing needles. Needle sharing, for instance for injecting drugs, can spread STIs as well as other infections, including hepatitis B and C.

Some groups of people are also at increased risk for STIs, including:

  • Younger individuals. Nearly half of all STI cases in the United States occur in people under the age of 25.
  • Men who have sex with men (MSM). Because the rates of HIV and other STIs are higher in this group, MSM are at increased risk.
  • Those who are HIV-positive

How are STIs diagnosed?

To diagnose an STI, your doctor will review your medical and sexual history, perform a physical exam, and run one or more diagnostic tests.

Your doctor may begin by asking you about your sexual history and your symptoms. During a physical exam, they may closely examine your genitals, mouth, throat, or other areas of your body for signs of an STI.

Laboratory testing is often necessary to confirm an STI diagnosis or for screening purposes. Tests may include:

  • Blood tests.
  • Urine test. Tests on urine samples may be used to diagnose some STIs.
  • Analysis of tissue and fluid samples collected from the genitals and/or sores. Various tests, including DNA analyses, may be run on samples to diagnose an STI.
  • Spinal tap (lumbar puncture). In some cases, cerebrospinal fluid (the fluid that surrounds the brain and spinal cord) is collected via a spinal tap, then analyzed for syphilis or herpes.

What routine screenings are recommended for STIs?

If you have any suspicion that you might have been exposed to or actually have an STI, see a doctor for testing. “We get a lot of patients who come in and want to be tested because they have a new partner or they might have been exposed or they are having symptoms,” Dr. Sheth says. “If they have one STI, we recommend testing for everything else as many of them, including chlamydia and gonorrhea, travel together.”

Screenings are often recommended for the following STIs:

  • Chlamydia and gonorrhea: Women under age 25 should get screened annually for chlamydia and gonorrhea. Women ages 25 and over who have multiple sex partners or have sex with someone with an STI should also get screened. Routine screenings aren’t currently recommended for men who have sex with women who are also at low risk for these STIs. Sexually active men who have sex with men should be screened annually or more frequently if they have more exposures (e.g., multiple partners).  
  • HIV: The CDC recommends that as part of routine health care, everyone aged 13 to 64 get an HIV test at least once. Around one in five HIV diagnoses are in youth and adolescents aged 13-24 years. What’s more, about 44% of young people with HIV don’t know they’re infected. Because of this, youth and adolescents should consider getting tested. The health agency also recommends that those at increased risk for HIV get tested more frequently. All pregnant women should also be tested.
  • HPV: The American Cancer Society (ACS) recommends that women between 30 and 65 receive a primary HPV test every five years. (The Food and Drug Administration (FDA) has approved certain tests as "primary HPV tests," which are in-office HPV tests that can be done on their own, without an accompanying Pap test.) If a primary HPV test is not available locally, the ACS recommends people with a cervix get a co-test—an HPV test done at the same time as a Pap test—every five years or a Pap test every three years.
    HPV is so common in women under age 30 that routine HPV testing is not recommended for this group. “We might pick up a transient infection that, in a normal, healthy person, the body will likely take care of it on its own,” Dr. Sheth explains. “But an important message is that 50% of cervical cancer is diagnosed in women who have been under-screened, meaning they haven’t had a Pap smear in the last five years.”
    Women between the ages of 21 and 29 should have Pap tests every three years, and HPV testing is recommended for those who receive abnormal Pap test results.
    HPV testing still has a long way to go. Though the virus can cause an infection in the mouth or throat, there are no FDA-approved tests to identify the virus in these locations. At the moment, there is also no FDA-approved HPV test for men.

How are STIs treated?

Treatment for STIs varies depending on the cause of the infection. For pregnant women, prompt treatment can prevent or reduce the risk of infection spreading to their baby. Options include:

  • Antibiotics: These are used to treat STIs caused by bacteria. This category includes gonorrhea, syphilis, and chlamydia. Antibiotics are also used to treat trichomoniasis, which is caused by a parasite. Most of the medicines are oral, though intramuscular injections are used for gonorrhea (which has a growing problem of antibiotic resistance) and for syphilis.
  • Antiviral drugs: STIs caused by some viral infections can't be cured, but they can still be managed with antiviral drugs. For HIV, antiviral drugs can control the virus and allow people to live longer, healthier lives. For patients with HIV, effective therapies are available that can reduce the amount of HIV in the blood to "undetectable levels," meaning the viral load is so low that it cannot be detected by lab tests. When HIV is undetectable in the blood, the virus cannot be sexually transmitted to others. This is often referred to by the phrase "undetectable = untransmittable," or U=U. For herpes, antiviral drugs can lower the risk of recurrences and reduce—but not eliminate—the risk of transmitting the virus to others.

How can STIs be prevented?

There are several ways to reduce the risk of getting an STI or transmitting one to someone else, including:

  • Abstinence. Not having sex is the most effective way to avoid getting an STI.
  • Use condoms when having vaginal, anal, and oral sex.
  • Get vaccinated against HPV and hepatitis B. The CDC recommends that everyone between 11 and 26 years of age get vaccinated against HPV, though vaccinations may be given starting at age 9. Adults up to age 45 may also get the vaccine after discussing the matter with their doctor. HPV vaccines protect against many strains of HPV known to cause cancer if it is administered before an individual is infected by those strains.
  • Have fewer sex partners. Having more sex partners increases the chances of getting an STI.
  • Get tested for STIs. Getting tested and sharing STI status with sex partners can help reduce the transmission of infections.
  • Get treatment. People who have an STI should seek treatment. Many STIs can be cured, thus preventing transmission. Treatment, even for STIs that cannot be cured, can also reduce the chances of transmitting an STI to sex partners.

What makes Yale Medicine unique in its approach to STI treatment?

Yale Medicine clinicians and researchers are working to advance the prevention and treatment of STIs. The Yale AIDS Care Program offers primary and consultative medical care, counseling and testing, social services, and patient and family support groups for adults, adolescents, and children with HIV. Yale also offered Pre-Exposure Prophylaxis (PrEP) to HIV negative patients who are at risk for acquiring HIV along with concomitant STI screening.

“Our clinic provides state-of-the-art care for people living with HIV/AIDS, as well as for those wishing to remain HIV negative,” says Yale Medicine infectious diseases specialist Dana Dunne, MD, MHS. “Our wrap-around services include on-site testing and counseling, as well as a rapid HIV start of antiretroviral treatment for those found to be newly HIV positive on screening. Our patients have access to the newer injectable antiretroviral medications for HIV treatment or prevention, as well as access to clinical trials.”

Yale’s Center for Interdisciplinary Research on AIDS (CIRA) is New England's only National Institute of Mental Health-funded AIDS research center. CIRA brings together scientists from 25 disciplines to support research that combines behavioral, social, and medical approaches to HIV prevention and treatment.

For chlamydia, Yale is involved in a National Institutes of Health (NIH) project to test a potential vaccine.

“For society, focusing on prevention is the best approach, and we are seeing a lot of exciting advances in terms of vaccines for STIs,” Dr. Sheth says.