Chlamydia (kluh-MID-e-uh) is a common sexually transmitted disease.

Sexually transmitted diseases are infections spread mainly by contact with genitals or bodily fluids. Also called STDs, STIs or venereal disease, sexually transmitted infections are caused by bacteria, viruses or parasites.

Chlamydia is caused by Chlamydia trachomatis (truh-KOH-muh-tis) bacteria and spread through oral, vaginal or anal sex.

You might not know you have chlamydia because many people don't have symptoms, such as genital pain and discharge from the vagina or penis. Chlamydia trachomatis affects mostly young women, but it can occur in both men and women and in all age groups.

It's not difficult to treat, but if left untreated it can lead to more-serious health problems.

Early-stage Chlamydia trachomatis infections often cause few symptoms. Even when symptoms occur, they're often mild. That makes them easy to overlook, which is why regular screening is important.

Symptoms of Chlamydia trachomatis infection can include:

• Painful urination.
• Vaginal discharge.
• Discharge from the penis.
• Painful vaginal sex.
• Vaginal bleeding between periods and after sex.
• Testicular pain.

Depending on a person's sexual activity, Chlamydia trachomatis can infect the eyes, throat or rectum.

Eye infections, called conjunctivitis, cause the inside of the eyelid to be red and irritated. In the throat, an infection may have no symptoms, or a person may have a sore throat. An infection in the rectum may have no symptoms or may cause rectal pain, discharge or bleeding.

When to see a doctor

See your healthcare professional if you have a discharge from your vagina, penis or rectum, or if you have pain during urination. Also, see your healthcare team if you learn your sexual partner has chlamydia. Your healthcare professional will likely prescribe an antibiotic even if you have no symptoms.

The Chlamydia trachomatis bacterium is most commonly spread through vaginal, oral and anal sex. It also is possible for the bacterium to spread in pregnancy, during delivery of the baby. Chlamydia can cause pneumonia or a serious eye infection in the newborn.

People who have sex before age 25 are at higher risk of chlamydia than are older people. That's because younger people are more likely to have more than one risk factor.

Risk factors for chlamydia include:

• Not using a condom or incorrect condom use.
• Less use of health services to prevent and treat sexually transmitted infections.
• New or multiple sex partners.
• Changing sex partners before learning about a chlamydia infection.

Chlamydia trachomatis can be associated with:

• Pelvic inflammatory disease, also called PID. PID is an infection of the uterus and fallopian tubes. Severe infections might require care in the hospital. PID can damage the fallopian tubes, ovaries and uterus, including the cervix.
• Infection near the testicles. A chlamydia infection can inflame the coiled tube located beside each testicle, called the epididymis. The infection can result in fever, scrotal pain and swelling.
• Prostate gland infection. Rarely, the chlamydia bacteria can spread to the prostate gland. Prostatitis can cause pain during or after sex, fever and chills, painful urination, and lower back pain.
• Infections in newborns. The chlamydia infection can pass from the vaginal canal to your child during delivery, causing pneumonia or a serious eye infection.
• Ectopic pregnancy. This occurs when a fertilized egg implants and grows outside of the uterus, usually in a fallopian tube. The egg needs to be removed to prevent life-threatening complications, such as a burst tube. A chlamydia infection increases this risk.
• Infertility. Chlamydia infections can cause scarring and obstruction in the fallopian tubes, which might lead to infertility.
• Reactive arthritis. People who have Chlamydia trachomatis are at higher risk of developing reactive arthritis, also known as Reiter syndrome. This condition typically affects the joints, eyes and urethra — the tube that carries urine from the bladder to outside of your body.

The surest way to prevent chlamydia infection is to abstain from sexual activities. Short of that, you can:

• Use condoms. Use a male latex condom or a female polyurethane condom during each sexual contact. Condoms used properly during every sexual encounter lower but don't eliminate the risk of infection.
• Limit your number of sex partners. Having multiple sex partners puts you at a high risk of contracting chlamydia and other sexually transmitted infections.
• Get regular screenings. If you're sexually active, particularly if you have multiple partners, talk with your healthcare professional about how often you should be screened for chlamydia and other sexually transmitted infections.

A medicine called doxycycline may be an option to prevent infection among people at higher risk than average of getting chlamydia. Higher risk groups include men who have sex with men and transgender women.

Taking doxycycline within 3 days of sexual activity lowers the risk of an infection with the bacteria that cause chlamydia. Your healthcare professional can prescribe doxycycline and any testing you need while taking the medicine.

The Centers for Disease Control and Prevention recommends chlamydia testing for anyone with chlamydia symptoms. Regardless of symptoms, talk to your healthcare team to find out how often you should be screened for chlamydia. In general, some groups are screened more often than others, such as:

• Sexually active women age 25 or younger. The rate of chlamydia infection is highest in this group, so a yearly screening test is recommended. Even if you've been tested in the past year, get tested when you have a new sex partner.
• Pregnant people. Chlamydia screening may be offered during the first prenatal exam. If you have a high risk of infection, get tested again later in your pregnancy. You are at high risk if you are younger than age 25, have a new sex partner or have a sex partner who might be infected.
• People at high risk. People who have new or multiple sex partners or men who have sex with men should consider more frequent chlamydia screening. Other markers of high risk are current infection with another sexually transmitted infection and possible exposure to an STI through an infected partner.

Screening and diagnosis of chlamydia is relatively simple. Tests include:

• A urine test. A sample of urine is analyzed in the laboratory for presence of this infection. This can be done for males and females.
• A swab. A sample from the cervix, vagina, throat or anus is collected on a swab for testing. From the cervix, a member of your healthcare team collects a sample of the discharge from the cervix on a swab for testing. This can be done during a routine Pap test. For a swab from the vagina, either you or the healthcare professional can do the swab. For males and females, depending on sexual history, a swab may be taken from the throat or the anus.

You can get tested by a healthcare professional or use a test available without a prescription. If you've been treated for an initial chlamydia infection, you should be retested in about three months.

Chlamydia trachomatis is treated with antibiotics. You might receive a one-time dose, or you might need to take the medicine daily or multiple times a day for seven days.

In most cases, the infection clears up within 1 to 2 weeks after you take the antibiotic. But you can still spread the infection at first. So avoid sexual activity from when you start treatment until all your symptoms are gone.

Your sexual partner or partners from the last 60 days also need screening and treatment even if they don't have symptoms. Otherwise, the infection can be passed back and forth between sexual partners. Make sure to avoid sexual contact until all exposed partners are treated.

Having chlamydia or having been treated for it in the past doesn't prevent you from getting it again.

If you think you have a sexually transmitted infection, such as Chlamydia trachomatis, see a healthcare professional.

What you can do

Before your appointment, prepare to answer the following questions:

• When did your symptoms begin?
• Does anything make them better or worse?
• What medicines and supplements do you take regularly?

You also might want to prepare a list of questions to ask your healthcare professional. Sample questions include:

• Should I be tested for other sexually transmitted infections?
• Should my partner be tested or treated for chlamydia infection?
• Should I avoid sex during treatment? How long should I wait?
• How can I prevent chlamydia infection in the future?

What to expect from your doctor

Your healthcare professional is likely to ask you a number of questions, such as:

• Do you have a new sexual partner or multiple partners?
• Do you use condoms consistently?
• Do you have pelvic pain?
• Do you have pain while urinating?
• Do you have sores or unusual discharge?