The Most Common Sexually Transmitted Infections (STIs)

Infections spread by sexual activity are known as sexually transmitted infections (STIs). An estimated 20% of Americans have an STI, according to the Centers for Disease Control and Prevention (CDC).

STI vs. STD: What's the difference?

According to the CDC, the term STI is used interchangeably with sexually transmitted disease (STD). However, in its 2021 treatment guidelines for STIs, the CDC says the bacterium, virus, fungus or protozoan causing the infection is an STI, and the disease resulting from the infection is an STD.

STI symptoms

Symptoms of an STI depend upon the illness. STIs may show no immediate symptoms, or symptoms may be so mild that they go unnoticed. People may also experience moderate or severe symptoms as well.

For example, human papillomavirus (HPV), the most common STI, often causes no symptoms, but some people may develop cauliflower-like bumps or warts on their genitals, and others, years after exposure, may develop cancer.

STI symptoms may also vary by gender. For example, in men, gonorrhea may cause a burning feeling while urinating and a yellow, green or white substance to leak from the penis. In women, there may be no noticeable symptoms, or they may be mistakenly thought to be from a urinary tract infection or another vaginal infection.

STI testing

If you're having sex, you're at risk of getting an STI and spreading it to others. You may already have one and not know it as well, as some STI symptoms may be minor or nonexistent, so it is important to get tested to see if you're infected. You should also be retested if you're sexually active, as reinfection is possible.

The CDC recommends speaking with a health care provider about your sexual history and discussing what tests may be appropriate. Many health clinics do tests for low or no fees. In addition, drugstores may sell kits to test for some STDs, but you should always follow-up with your health care provider after taking any test.

The CDC suggests the following groups be tested:

Everyone 13 to 64 years old should be tested at least once for human immunodeficiency virus (HIV).
Sexually active women who are 25 years old or younger should be tested annually for gonorrhea and chlamydia, and those 25 years old and older who have a new sex partner or more than one partner should also be tested annually for these two infections.
Pregnant people should be tested early in pregnancy for HIV, syphilis and hepatitis B and C, and possibly gonorrhea and chlamydia as well, if they're at risk.
Men who have sex with men should be tested annually or more often for gonorrhea, syphilis, chlamydia and HIV. If infected with HIV, they should be tested at least annually for hepatitis C.
People who share injectable drug equipment should get tested annually or more often for HIV.

The only sure way to not get an STI is abstinence. According to the CDC, you can lower the odds of catching an STI through:

Good hygiene practices
Using condoms
Staying up to date with vaccinations, such as the hepatitis B and HPV vaccines
Avoiding sex when you or a partner is infectious
Limiting sex partners
Not sharing injectable drug equipment

Be aware that some sex practices may increase the risk of infection. For example, anal sex may raise the risk because the lining of the rectum tears easily, according to the American College of Obstetricians and Gynecologists (ACOG).

The most common STIs, their symptoms and treatment

Some of these infections may be cured, while others may be lifelong or disappear with time. Treatment varies with each STI, and you generally can't undo the damage an STI has caused. Several of the most common STIs may wreak serious — even deadly — harm, if not treated. Reinfection is also possible with most STIs.

The most common STIs include:

Human papillomavirus (HPV)

HPV is a viral infection and the most common STI. In 2018, it was estimated that there were about 43 million HPV infections in the United States, according to the CDC. Most people contract HPV soon after becoming sexually active, according to the U.S. National Library of Medicine.

HPV may be spread by:

Oral sex
Anal sex
Vaginal sex
Intimate skin contact

More than 200 strains exist of this group of viruses.

HPV symptoms

In most cases, the infection disappears within 2 years and causes no symptoms. Some low-risk strains may cause warts to develop in or near the genitals, anus, mouth and throat. High-risk strains may remain in the body and cause cancer of the genitals, anus and throat years or decades after exposure. HPV is the leading cause of cervical cancer, according to the U.S. National Library of Medicine.

HPV treatment

There's no cure or treatment for HPV infection, but warts may be removed with topical medications, freezing or burning. Cancer treatments are similar to those for other cancers. Women may get an HPV test, often along with a Pap smear, that examines tissue from the cervix for abnormal cells.

Chlamydia

Chlamydia is the most common STI caused by a bacterium, according to the CDC. The germ responsible is Chlamydia trachomatis.

You may contract chlamydia via:

Vaginal sex
Anal sex
Oral sex

Pregnant people may give it to their babies during birth.

Chlamydia symptoms

Most infected people don't have symptoms. In people who do show symptoms, it may take weeks after exposure for symptoms to be noticed.

In women, chlamydia may infect the cervix, uterus and fallopian tubes, resulting in pelvic inflammatory disease (PID), possibly causing long-term pelvic pain and infertility. For pregnant people, it may raise the risk of premature birth and cause pneumonia or conjunctivitis in newborn babies. It also increases the chances of ectopic pregnancy, which may be fatal. PID may affect the connective tissue surrounding the liver, and it may also infect the urethra, resulting in painful urination and pus in urine.

In men, infection may result in inflammation of the urethra, with pain during urination and a discharge of fluid from the penis. Painful, swollen testicles are also possible.

For both sexes, infection of the rectum may cause pain and discharge, and possibly bleeding. Women and men may both spread infection to their eyes, causing conjunctivitis.

Chlamydia treatment

Chlamydia may be cured with antibiotics, either with a single-day or seven-day course. People should abstain from sex for seven days after a single-day course or until they complete a seven-day course and no symptoms remain.

Genital herpes

Genital herpes is an infection with either herpes simplex virus type 1 (HSV-1) or herpes simplex virus type 2 (HSV-2). The latter is more associated with genital infections. Infection with HSV-1 causes a rash commonly known as cold sores or fever blisters.

More than half of Americans get a mouth infection with HSV-1 in childhood or in their 20s, according to the U.S. National Library of Medicine.

While HSV-1 infection typically affects the area surrounding the mouth, it may be spread to the genitals during oral sex. The presence of blisters or a rash from either HSV type increases the risk of transmission during sex, but the virus may also spread from normal-looking skin or mucosa, as well as by oral and genital secretions. In fact, transmission likely most often happens when the infected partner has no visible lesions or doesn't realize he or she is infected.

Genital herpes symptoms

Many people with genital herpes infections don't have symptoms or mistake them for another skin problem. The first outbreak of blisters is typically the worst, and occurs a few days after exposure. Subsequent outbreaks may be preceded by pain in the genitals or tingling and pain in the legs and buttocks. People with weakened immune systems may develop severe ulcers on their genitals. Pregnant women with genital herpes may infect their child, a situation that may be fatal to the newborn. For a woman with an active outbreak during labor, a cesarean delivery is recommended.

Genital herpes treatment

There is no cure for genital herpes, nor vaccine to prevent catching it. Antiviral medicines may shorten or prevent outbreaks.

Gonorrhea

Gonorrhea is the second most common STI caused by a bacterium in the United States, the CDC says. (Chlamydia is the most common bacterial STI.)

Infection with Neisseria gonorrhoeae targets reproductive organs in women and the urethra and rectum in men and women. It may also occur in the mouth, throat and eyes.

Gonorrhea is spread by:

Vaginal sex
Anal sex
Oral sex

Infected mothers may spread it to their babies during birth. Infected babies may go blind, have joint problems and possibly die.

Gonorrhea symptoms

Many infected people show no symptoms. In women, mild symptoms may be mistaken for other urinary or vaginal infections. Women may have pain or difficulty during urination, vaginal bleeding between regular menstrual periods and more vaginal discharge.

Within two weeks of exposure, men may experience uncomfortable urination or have a penile discharge that's white, yellow or green.

Both women and men with rectal infections may have no symptoms or may have an anal discharge, itching or discomfort, bleeding and pain during bowel movements. Throat infections most often produce no symptoms, but a sore throat is possible.

If left untreated, gonorrhea may cause PID in women, which may result in pelvic pain and internal abscesses, as well as potentially increase the chances of infertility and ectopic pregnancy.

In men, gonorrhea may inflame the epididymis at the rear of the testicles, potentially causing infertility.

In men and women, an untreated infection may enter the bloodstream and spread throughout the body, causing dermatitis, arthritis and inflammation of the membrane surrounding the tendons (tenosynovitis). Such a widespread infection, called disseminated gonococcal infection, or DGI, may be fatal.

Gonorrhea treatment

The CDC recommends a single injection of the antibiotic ceftriaxone, which can cure the infection in most cases. However, treating gonorrhea successfully with antibiotics is getting more difficult, as the microbe is becoming resistant to the class of antibiotics known as cephalosporins, which includes ceftriaxone.

Syphilis

Syphilis is caused by a bacterium, Treponema pallidum, and cases have been on the rise since a low point in 2000-2001, according to the CDC.

It is spread during sex by contact with a syphilitic sore called a chancre. These sores may be on or near the genitals, anus and mouth. Syphilis may also spread from a pregnant person to their baby, and children infected this way have what's called congenital syphilis. Mothers who have untreated syphilis have up to a 40% chance of delivering a stillborn baby, the CDC says.

Syphilis symptoms

If untreated, syphilis progresses through several stages. Initially, it may take 10-90 days for symptoms to appear. In the primary stage, a single chancre, but sometimes several, occur near the location of exposure. The sore is painless and may be overlooked. It will heal within 3-6 weeks, whether or not treatment is given.

In the secondary stage of untreated syphilis, skin rashes and lesions in the vagina, anus and mouth occur. The rashes aren't typically itchy, and may be faint, or rough and reddish in color. Lesions in the mouth, groin and armpits, called condyloma lata, may appear, and may be raised, large and white or gray. During this stage, other symptoms may include aches, fatigue, fever, swollen lymph nodes, weight loss and hair loss. As with the chancres in the primary stage, these symptoms will eventually disappear whether the infection is treated or not.

After the secondary stage, the disease may enter a symptom-free latent stage that may last for years.

However, during any stage, the infection may attack the eyes, ears or nervous system. These conditions are known, respectively, as ocular syphilis, otosyphilis and neurosyphilis. Depending upon the affected area, symptoms may include vision changes and eye pain, hearing loss and problems with balance, motor problems and paralysis, or mental difficulties, personality changes and dementia.

Rarely, long-term untreated syphilis may attack one or more sites or systems in the body. It may happen 10-30 years after initial exposure. Called tertiary syphilis, it may harm the nervous and circulatory systems, as well as bones, joints and eyes.

Syphilis treatment

Treatment with antibiotics varies with the stage or type of syphilis infection. For primary, secondary and early latent infections, a single injection of benzathine penicillin is given. Late latent stage or latent stage of unknown length infections are treated with one dose weekly for 3 weeks. For ocular syphilis, otosyphilis and neurosyphilis, aqueous crystalline penicillin G is given as an intravenous infusion continuously or every 4 hours for 10-14 days.

People receiving treatment should not have sex until all sores are healed.

Mycoplasma genitalium (Mgen)

Mycoplasma genitalium (Mgen) is an STI named for the responsible bacterium. It is spread through vaginal and anal sex. Researchers don't yet know if it is spread by oral sex.

Mgen infects the urethra and cervix and may be associated with:

PID
Infertility
Premature delivery
Spontaneous abortion

Mgen symptoms

In women, inflammation of the cervix often causes no symptoms, but there may be uncomfortable urination, pelvic pain and vaginal discharge or itching.

In men, inflammation of the urethra also may be asymptomatic, or there may be pain and difficulty urinating and a discharge from the penis. Infections of the rectum and throat may have no symptoms.

Mgen treatment

A two-step antibiotic process is used to treat Mycoplasma genitalium. Doxycycline is given followed by either azithromycin or moxifloxacin. Treatment with just doxycycline or azithromycin cures infection in only half or fewer cases. Moxifloxacin alone used to be nearly 100% effective, but cure rates for it have dropped, the CDC reports.

Human immunodeficiency virus (HIV)

HIV is the viral infection that causes acquired immunodeficiency syndrome (AIDS), a potentially fatal condition.

HIV is spread by:

Vaginal sex
Anal sex
Injectable drug use
Blood
Breast milk

You are more likely to contract or spread HIV if you have another STI. Infections with genital herpes, gonorrhea and syphilis are associated with HIV infection or transmission. A sore or opening in skin may allow HIV to more easily enter the body. Also, behaviors that led to contracting a prior STI may lead to getting HIV.

HIV symptoms

Symptoms may develop 2-4 weeks after exposure, and last as long as a few weeks, or there may be no symptoms. Initial symptoms may be similar to those of the flu, and include:

Aches
Fever
Night sweats
Fatigue
Sore throat
Mouth ulcers
Swollen lymph nodes

This is stage 1, or acute HIV infection, when there are many viral particles in your blood and you are very contagious.

If untreated, stage 2, or chronic HIV infection, follows. The virus keeps reproducing, but there may be no symptoms. This stage may last 10 years or longer. If untreated, stage 3, or AIDS, follows.

People with AIDS may have high amounts of the virus in them, and their immune systems are damaged, making them vulnerable to catching other infections and diseases that their bodies might normally be able to fight off. If not treated, they may die within 3 years.

HIV treatment

HIV can't be cured, but medical advances have changed it from a death sentence to a treatable chronic disease. Antiretroviral therapy (ART) lowers the amount of the virus in your body to a point it may be undetectable. These medications may be given as pills or injections. If viral levels drop to undetectable, it prevents spreading the infection through sex. It also probably reduces the chances of spreading it through shared injectable drug use, though more research is needed to determine by how much. It also reduces the chances of spreading HIV through breast milk, though breastfeeding is not recommended.

Unlike other STIs, the odds of contracting HIV may be reduced by taking medications known as PrEP (pre-exposure prophylaxis). There are currently two pills (Truvada® and Descovy®) and a shot (Apretude) approved for this use.

Trichomoniasis

Trichomoniasis, called "trich," is caused by a protozoan, Trichomoniasis vaginalis, and is the most common STI that can be cured.

It is usually spread during vaginal sex, and affects the vulva, vagina, cervix and urethra in women and the urethra in men. Infections in other areas, such as the mouth or anus, are unusual. Untreated pregnant people are more likely to have premature births and low-weight infants.

Trich symptoms

An estimated 70% of infected people don't have symptoms, according to the CDC.

In people who do experience symptoms, they may appear 5-28 days after exposure, or even later. They may also go away and return.

In infected women, genitals may be red, itchy and irritated, and there may be pain during urination. There may be a vaginal discharge that smells fishy and ranges in color from clear to white, yellow or green.

In men, the urethra may feel itchy or irritated, and there may be a discharge from the penis, as well as a burning sensation during urination and ejaculation.

For infected people, having sex may be uncomfortable.

Trich treatment

Trichomoniasis may be cured with antibiotics. Recommended treatment for women is metronidazole twice daily for a week, and for men the treatment is a larger single dose of metronidazole.

Hepatitis A, B and C

Hepatitis A, B and C are viruses that attack the liver. Hepatitis A and B may be spread through sex and sharing injectable drug equipment. Hepatitis C may also be spread through injectable drug use, but the chances are less common for it to be transmitted through sex. There are vaccines to prevent contracting hepatitis A and B, but not for hepatitis C.

Hepatitis types and their symptoms and treatment

Hepatitis A is quite contagious. The infection may cause illness for weeks or months, but people usually fully recover. Symptoms include a yellow tint to eyes or skin (jaundice), nausea, diarrhea, fever, joint pain, fatigue, dark urine and light-colored stools. Not everyone has symptoms. Treatment is typically rest, fluids and a healthy diet. Severe cases may need hospitalization.
Hepatitis B may be spread through childbirth, in addition to sex and intravenous drug use. Symptoms are similar to hepatitis A. It may be a short illness, but for infants and a few adults, it may become chronic with serious problems, including liver cancer and liver disease. According to the CDC, about 90% of HBV-infected infants develop chronic disease. Infection is treated with antiviral medications. The disease may also reappear after being inactive, a situation called HBV reactivation.
Hepatitis C is primarily spread through blood, such as sharing injectable drug equipment. Some infected people have a short illness, but more than 50% of people with hepatitis C develop chronic, serious illness, according to the CDC. People with hepatitis C often don't feel sick. Symptoms occur often when liver damage has already occurred. Treatments may cure more than 90% of infections in 2-3 months, according to the CDC. Treatments depend upon the genotype of the virus and whether cirrhosis exists.

References:

Centers for Disease Control and Prevention (CDC): Sexually Transmitted Diseases (STDs), Diseases and Related Conditions.

Centers for Disease Control and Prevention (CDC): Incidence, Prevalence and Cost of Sexually Transmitted Diseases in the United States.

Centers for Disease Control and Prevention (CDC): Sexually Transmitted Infections Treatment Guidelines, 2021.

Centers for Disease Control and Prevention (CDC): Human Papillomavirus (HPV), HPV Fact Sheet.

Centers for Disease Control and Prevention (CDC): Gonorrhea, Basic Fact Sheet.

Centers for Disease Control and Prevention (CDC): Sexually Transmitted Diseases (STDs), Which STD Tests Should I Get?

Centers for Disease Control and Prevention (CDC): Sexually Transmitted Diseases (STDs), Prevention.

American College of Obstetricians and Gynecologists (ACOG): How to Prevent Sexually Transmitted Infections (STIs).

U.S. National Library of Medicine MedlinePlus: HPV.

National Library of Medicine MedlinePlus: Human Papillomavirus (HPV) Test.

Centers for Disease Control and Prevention (CDC): Chlamydia, Detailed Fact Sheet.

Centers for Disease Control and Prevention (CDC): Genital Herpes, Detailed Fact Sheet.

National Library of Medicine MedlinePlus: Cold Sores.

Centers for Disease Control and Prevention (CDC): Gonorrhea, Detailed Fact Sheet.

Centers for Disease Control and Prevention (CDC): Syphilis, Detailed Fact Sheet.

Centers for Disease Control and Prevention (CDC): Mycoplasma genitalium (Mgen), Detailed Fact Sheet.

Centers for Disease Control and Prevention (CDC): HIV, Bodily Fluids That Transmit HIV.

Centers for Disease Control and Prevention (CDC): HIV/AIDS & STDs.

Centers for Disease Control and Prevention (CDC): HIV, About HIV.

Centers for Disease Control and Prevention (CDC): About PrEP.

Centers for Disease Control and Prevention (CDC): Trichomoniasis, Fact Sheet.

Centers for Disease Control and Prevention (CDC): Sexually Transmitted Infections Treatment Guidelines, 2021, Trichomoniasis.

Centers for Disease Control and Prevention (CDC): Viral Hepatitis, Sexual Transmission.

Centers for Disease Control and Prevention (CDC): Viral Hepatitis, Q&As for the Public, Overview.

Centers for Disease Control and Prevention (CDC): Viral Hepatitis, Hepatitis B.

Centers for Disease Control and Prevention (CDC): Viral Hepatitis, Hepatitis B, Q&As for Health Professionals.

Centers for Disease Control and Prevention (CDC): Viral Hepatitis, Hepatitis C.

American Association for the Study of Liver Diseases, Infectious Diseases Society of America: HCV Guidance: Recommendations for Testing, Managing and Treating Hepatitis C.

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