But that number is undercounted: Many people with STDs, also known as venereal diseases, go undiagnosed and untreated. The CDC estimates that nearly 20 million new sexually transmitted infections occur every year, accounting for almost $16 billion in healthcare costs annually.

Here’s what you need to know about how to spot, treat, and prevent STDs.

What Is the Definition of an STD?

“STDs are diseases that are passed from one person to another through sexual contact,” a representative of the CDC says.

What Are the Most Common STDs?

According to the CDC, some of the common STDs are chlamydia, gonorrhea, herpes simplex virus type 2 (HSV-2), human immunodeficiency virus (HIV), human papillomavirus (HPV), and syphilis. "Many of these STDs do not show symptoms for a long time," per the CDC, "but they can still be harmful and passed on during sex.”

What Are the Ways I Can Get an STD?

Virtually all STDs can be transmitted through anal, vaginal, or oral sex. In addition, some STDs can also be transmitted through close skin-to-skin contact, even if no intercourse occurs.

HPV, for example, can be spread through skin-to-skin touching. In addition, “Molluscum contagiosum, a viral skin disease, can be spread through sexual or casual contact, as can scabies, an itchy skin condition caused by a mite infestation. It is also possible to get scabies from an infected sleeping bag or bed,” says Edward W. Hook III, MD, an endowed professor of infectious disease translational research in the departments of medicine, epidemiology, and microbiology at the University of Alabama in Birmingham, who works with the CDC.

STDs don’t just affect the genital regions: “Oral herpes can be transmitted through oral and genital sex,” says Dr. Hook.

What Are the Signs That I Might Have an STD?

It is important to remember that STDs may have no symptoms. However, new vaginal or urethral discharge or a new rash after sexual contact should be evaluated by a medical professional.

When symptoms do occur, they can include the following:

Chlamydia Symptoms of chlamydia can include vaginal discharge in women, penile discharge in men, and burning during urination in men and women.

Gonorrhea Gonorrhea can cause thick, cloudy, or bloody discharge from the vagina or urethra, and pain or burning when peeing. If you have gonorrhea in your anus, it may cause itching in and around the anus, discharge from the anus, and pain when defecating. Gonorrhea in the throat may cause a sore throat.

Hepatitis B Acute hepatitis B can cause fever, fatigue, loss of appetite, nausea or vomiting, abdominal pain, dark urine, clay-colored bowel movements, joint pain, and jaundice (yellowing of the skin and whites of the eyes). Symptoms can appear anywhere from six weeks to six months following exposure to the hepatitis B virus. Chronic hepatitis B sometimes causes symptoms similar to acute disease.

Genital Herpes Signs of genital herpes typically include red bumps that develop into blisterlike sores in the genital area and sometimes on the buttocks or thighs. A new infection with HSV-2 — the virus that causes most cases of genital herpes — may also cause flulike symptoms, including fever, headache, feeling tired and achy, and swollen glands.

Oral Herpes Symptoms of oral herpes can include itching of the mouth or lips, sores or blisters on the lips or inside the mouth, and flulike symptoms such as fever, headache, body aches, and swollen glands.

HIV Early symptoms of HIV infection can resemble those of the flu: fever, headache, muscle aches, and sore throat. They may also include swollen lymph nodes, diarrhea, nausea and vomiting, a fungal infection of the mouth, and a rash on the abdomen, arms, legs, or face. If HIV goes untreated, later symptoms can include fatigue, weight loss, night sweats, joint pain, short-term memory loss, and recurrent infections.

HPV Most strains of HPV cause no symptoms and are detected only after abnormal cells are discovered during a Pap smear. However, some types of HPV cause genital warts, which appear as skin-colored or whitish growths on the genitals or anus.

Molluscum Contagiosum Often the only sign of this skin disease is pink or flesh-colored bumps with a dimple (indentation) in the center. It is most common in children, who typically get it from skin-to-skin contact or from shared towels or similar items. In adults, it can be sexually transmitted.

Pubic Lice Symptoms of pubic lice include itching in the genital area, tiny bugs in your pubic hair, and visible nits (eggs) on hair shafts. Pubic lice can also infest the hair on legs, armpits, eyebrows, eyelashes, and other facial hair such as mustaches and beards.

Scabies This skin infestation causes intense itching that is typically worse at night. It can also cause small red bumps or a rash and raised lines on the skin where the mites have burrowed.

Syphilis In the primary stage, syphilis causes a painless sore, or ulcer, at the location the bacteria entered the body, often in the genital area. In the secondary stage of syphilis, a rash may occur on the torso and elsewhere on the body.

Trichomoniasis Another common STD, trichomoniasis is a parasitic infection that can cause burning and itching in the genital area for men and women as well as painful intercourse. Trich can also cause a smelly discharge and painful or frequent peeing.

Can I Have an STD and Not Know It?

Yes. Many STDs don’t cause any symptoms, especially during the early stages, so the only way to know for sure if you have one is to get tested. Keep in mind that you can get an STD from having sex with someone who has no symptoms and may not know they have an STD.

Are Adolescents Disproportionately Affected?

The CDC reports that half of newly reported cases of STDs occur in people between the ages of 15 and 24, noting that young women’s bodies are biologically more susceptible to STDs.

“Chlamydia and HPV are common as soon as you become sexually active,” Hook says. “Gonorrhea and genital herpes peak in the late twenties to thirties.”

All STDs, though, including HIV, can and do occur at any age.

When and How Often Should I Get Tested?

“Women should be tested for chlamydia on a regular basis. Some gynecologists test for it automatically, but not all do,” Hook says.

Testing for chlamydia, gonorrhea, and trichomoniasis involves collecting either a urine sample or a swab from the vagina or penis.

For other STDs, including HIV, syphilis, and genital herpes, blood testing is most accurate.

To test for HPV, a sample of cervical or anal cells must be collected.

How often an individual needs to be tested for STDs depends on their level of risk for infection.

The CDC recommends the following for testing for chlamydia:

• Annual screening in sexually active women age 25 and younger and in older women who are at an increased risk for infection because of a new sex partner or multiple sex partners
• Annual screening in men who have sex with men, based on exposure history, with more frequent screening in people at the highest risk
• Screening in all pregnant women at their first prenatal visit
• Annual screening in sexually active people living with HIV

The CDC's recommendations for gonorrhea testing include the following:

• Annual screening in sexually active women who are at risk for infection, which includes women ages 25 and younger
• Annual screening in men who have sex with men, based on exposure history, with more frequent screening in people at the highest risk
• Screening in all pregnant women under age 25 and older women who are at an increased risk
• Annual screening in sexually active people living with HIV

The CDC's recommendations regarding screening for syphilis include these guidelines:

• Screening in all pregnant women at their first prenatal visit
• Annual screening in men who have sex with men
• Annual screening in sexually active people living with HIV

The CDC has additional recommendations for other STDs.

In all cases, more frequent screening or screening for additional STDs may be appropriate for certain individuals, depending on their risk factors, including sexual behavior and how common a particular disease is in their area.

Where Can I Get Tested for STDs?

Your family doctor or ob-gyn should be able to do STD testing. In some areas, other options include specialized STD or sexual health clinics.

You can find locations in your area that offer fast, free, and confidential testing using the CDC’s Get Tested tool.

How Can I Protect Myself From Getting an STD?

Abstaining from all sexual contact is the only way to reduce your risk of STDs to zero. But the following measures can also help:

• Having only one sexual partner, and ensuring that person is monogamous too, lowers your risk of STDs.
• Proper use of a condom every time you have sex greatly reduces the risk of all STDs.
• Getting vaccinated against hepatitis B is the best way to avoid this viral infection. The hepatitis B vaccine can be given to people of any age.
• Getting the HPV vaccine protects you against strains of the virus that cause genital warts and cervical and other cancers. However, the HPV vaccine is currently available only to people in their twenties or younger.

What Are the Treatment Options for Common STDs?

Some STDs are curable with medical treatment, while others can be managed to control symptoms and reduce the risk of transmission.

These STDs can be cured with antibiotics:

• Chlamydia
• Gonorrhea
• Syphilis
• Trichomoniasis

STDs that can be cured with insecticides include the following:

• Pubic lice
• Scabies

Minor surgical procedures such as cryosurgery (freezing) or laser surgery can treat certain STDs:

• Genital warts caused by HPV
• Molluscum contagiosum

STDs that can be managed with antiviral medication include these infections:

• Chronic hepatitis B
• Genital herpes
• HIV

When precancerous cells caused by HPV are found on the cervix, treatment may include watchful waiting or surgery to remove the abnormal tissue.

Does Having One STD Make You More Likely to Get Another?

“Having one STD does not predispose you to others, aside from the behavioral risks shared by all STDs,” says H. Hunter Handsfield, MD, a professor emeritus of medicine at the University of Washington Center for AIDS and STD in Seattle, who also served on the board of the American Sexual Health Association.

“However,” says Dr. Handsfield, “having genital herpes due to HSV-2 — but not HSV-1 — roughly doubles the risk of HIV if sexually exposed to the virus.”

What Happens If STDs Are Left Untreated?

“The longer something goes untreated, the more likely it is to cause complications,” Hook says. “Left untreated, infections from gonorrhea and chlamydia can travel from a woman’s cervix to her uterus and fallopian tubes, which may cause infertility.”

Untreated syphilis "can become neurosyphilis, causing neurological disease,” says Suzanne Fenske, MD, an assistant professor of obstetrics, gynecology, and reproductive science at the Icahn School of Medicine at Mount Sinai in New York City. Late-stage complications of syphilis also include aortic aneurysm and other cardiovascular problems.

Untreated HIV can impair the immune system’s ability to fight off infections and diseases, leading to so-called opportunistic infections, neurological complications, and sometimes cancer.

Untreated chronic hepatitis B can lead to liver failure, cirrhosis, liver cancer, and death.

Failing to treat STDs can have risks for future generations as well. Women who have herpes, chlamydia, gonorrhea, or syphilis can transmit the infection to their baby at birth, which is why testing for STDs — and following safer sex recommendations — during pregnancy is so important.

HIV can also be passed from mother to child during pregnancy, labor, or delivery, but the risk of transmission can be reduced if the mother takes antiretroviral drugs throughout her pregnancy and the baby receives HIV medicines for several weeks after birth.

Does the Number of Sexual Partners Affect My Chances of Getting an STD?

“As a broad rule, more partners means higher risk,” Handsfield says. “However, there are innumerable exceptions. Some people have 100 or more partners per year and remain free of most if not all STDs. Others have one or two partners and multiple infections over time.”

The bottom line is to use condoms or dental dams (small sheets of latex) to lower your risk and discuss STD testing with your medical provider if you are sexually active.

Can STDs Recur?

A curable STD that has been treated correctly will not recur, but it is possible to get the same STD again. In fact, it’s fairly common. To avoid getting the same STD again, your sexual partner — or partners — must also be treated. And to avoid getting the same or other STDs after treatment, practice safer sex, including using condoms each time you have sex.

The good news is that getting infected more than once does not appear to cause more long-term complications.

“In women, a second or third infection with chlamydia may raise the risk of complications like pelvic inflammatory disease,” says Handsfield. “But for the most part, people with multiple episodes of STDs do not fare any worse than after the first infection.”

Chlamydia: Often a Silent Disease

Many people don't realize they have chlamydia because they don't experience symptoms. That's why the condition is often called a silent infection. But untreated chlamydia can cause serious complications in both men and women, including ectopic pregnancy in women. To prevent such complications, young women and men at high risk of chlamydia are advised to get tested for it yearly.

Learn More About Chlamydia

Genital Herpes: A Lifelong Infection

An easily transmitted disease, genital herpes is caused by a virus that stays in the body for life. The infection can cause painful sores to form on the genitals, thighs, or buttocks, but the severity of symptoms ranges widely from no symptoms at all to recurring sores, painful urination, and flulike symptoms. Medication can prevent or shorten outbreaks, as well as lower the chance of passing the virus to a sexual partner.

Learn More About Genital Herpes

Gonorrhea: Still Curable, but Antibiotic Resistance Is a Problem

Gonorrhea is highly contagious and, like chlamydia, may cause mild or no symptoms. If left untreated, though, it can cause serious complications, including pelvic inflammatory disease in women and epididymitis in men. While gonorrhea is curable, some strains of the bacteria that cause it have become resistant to most antibiotics. Currently, giving two antibiotics simultaneously is the preferred treatment in the United States.

Learn More About Gonorrhea

Hepatitis B: Disease of the Liver

Hepatitis is an inflammation of the liver, and the hepatitis B virus is one of several viral causes of hepatitis. There are also nonviral causes. In the United States, infants have been routinely vaccinated against hepatitis B since 1991, and the rate of acute hepatitis B in the United States has plummeted since then. Worldwide, however, chronic hepatitis B infection is common.

Learn More About Hepatitis B

HIV: Incurable but Preventable

HIV is a virus that attacks the body's immune system, impairing its ability to fight off infections and diseases over time. The final stage of HIV infection is called acquired immunodeficiency syndrome, or AIDS. AIDS is a life-threatening disease. You can reduce your risk of getting sexually transmitted HIV by using condoms correctly and consistently.

Learn More About HIV

HPV: A Growing Cause of Cancer

Just about everyone who is or has been sexually active has had HPV at some point. In most cases, the virus clears up on its own, but when it doesn’t, it can cause genital warts and a variety of types of cancers, depending on which strain of the virus you have. The Gardasil 9 (HPV 9-valent) vaccine protects against both warts and cancer and is approved for adolescents and young adults.

Learn More About HPV

Molluscum Contagiosum: May Clear on Its Own

This viral infection can be sexually transmitted or spread through contact with infected skin or contaminated objects, such as towels and toys. It can also be spread from one area of the body to another by touching a lesion then an unaffected area of skin. In many cases it goes away on its own, but treatments are available if it doesn’t.

Learn More About Molluscum Contagiosum

Syphilis: Life-Threatening If Not Treated

Syphilis is a bacterial infection that is growing more common in the United States, particularly among men who have sex with men. It can be cured with penicillin or, in the case of a penicillin allergy, other antibiotics. Left untreated, syphilis can affect the heart and brain and be life-threatening. Syphilis that is passed from mother to infant (congenital syphilis) is also a severe, life-threatening condition.

Learn More About Syphilis