Chlamydia transmission: how common is chlamydia and how is it spread?

Mike Bohl, MD, MPH, ALM - Contributor Avatar

Written by Michael Martin 

Mike Bohl, MD, MPH, ALM - Contributor Avatar

Written by Michael Martin 

last updated: May 21, 2021

4 min read

The U.S. has some of the highest rates of sexually transmitted infection (STI) rates in the industrialized world. And these numbers have been steadily increasing, with a new record set in each of the last several years. Since chlamydia is the biggest culprit, we’re going to examine the role chlamydia plays in rising rates of STIs in the U.S. 


Men’s healthcare, without the waiting room

What is chlamydia?

Chlamydia is by far the most common notifiable STI, with 1.7 million cases diagnosed in 2017. That's a 22% increase in reported chlamydia cases since 2013, according to a report by the Centers for Disease Control and Prevention (CDC) (CDC, 2018).

Chlamydia is an STI caused by Chlamydia trachomatis bacteria. It's passed person-to-person through sexual contact with an infected person's penis, vagina, mouth, or anus. Ejaculation doesn't have to occur for it to be transmitted. Chlamydia can also be spread from mother to infant during childbirth.

According to the CDC, only 10% of men and 5% to 30% of women who test positive for a chlamydia infection will develop symptoms (CDC, 2021-a). 

In women, chlamydia can cause pain, irritation, a vaginal discharge, or pain while urinating. If left untreated, chlamydia can spread to the cervix, uterus, ovaries, and fallopian tubes, causing pelvic inflammatory disease (PID), signified by abdominal pain or pelvic pain. It can have serious implications for women's fertility, causing scarring or blockages in the fallopian tubes. These blockages can cause infertility or an ectopic pregnancy, which can be fatal (Hsu, 2019).

In men, chlamydia complications are less severe. It can lead to urethritis (a swelling in the urethra), which causes pain while urinating and discharge. Untreated chlamydia can lead to epididymitis, swelling in the tubes in the back of the testicles, which can cause pain (Hsu, 2019).

Both women and men can be infected with chlamydia in the rectum, which can cause pain, discharge, or bleeding (Hsu, 2019). 

Symptoms do not show up immediately when they do develop. It usually takes around 5–14 days after exposure to chlamydia for symptoms to begin. If you have symptoms or develop complications, chlamydia is considered a sexually transmitted disease (STD) (Hsu, 2019).

How does chlamydia spread?

Chlamydia spreads through sexual contact. Where you're infected will depend on the types of sexual activities you enjoy. You can get chlamydia through anal sex, oral sex, or vaginal sex with an infected sex partner, even if ejaculation does not occur. However, you can't get chlamydia through kissing or sharing cups with somebody who has chlamydia.

Around 50–70% of infected pregnant women pass chlamydia to their newborn during birth, potentially causing pneumonia or conjunctivitis in the child (Hammerschlag, 2018).

Why are chlamydia rates rising?

According to the CDC, the U.S. has just set another all-time high regarding the overall STI prevalence rates, including chlamydia, gonorrhea, and syphilis (CDC, 2021-b). Chlamydia rates are highest in young adult women. However, ethnicity may also be a factor—the incidence of chlamydia is nearly six times higher for African Americans than Caucasians. It is also seen in men who have sex with men (MSM).

The chlamydia statistics are concerning. From 2011–2014, researchers estimate that the number of cases of chlamydia rose 11% in women and almost 39% in men. Drug use, poverty, stigma, and unstable housing may reduce sexual health and medical care access. Other potential reasons for the rising rates of chlamydia, along with other STIs, include (CDC, 2019): 

1. A rise in condomless sex

The development of effective treatments for HIV—and prevention in the form of PrEP—has led to a decline in condom use. At the same time, location-based dating apps have made sex more easily accessible (Holt, 2018). 

PrEP stands for Pre-Exposure Prophylaxis—it is a pill you take each day to protect you against getting HIV. There's also a type of medication called PEP, which stands for Post-Exposure Prophylaxis. As the name suggests, PEP is taken after a potential exposure to HIV to prevent becoming infected. Both are incredible advancements in reducing HIV transmission rates.

Unfortunately, they may also be discouraging condom use, which drives increases in STI transmission rates, including chlamydia (Holt, 2018).

 2. Better detection and screening

In most cases, chlamydia causes no symptoms—but it can cause serious complications in women if left untreated. Chlamydia rates are highest among young women; the CDC says one in 20 sexually active females between the ages of 14 and 24 are believed to be infected (CDC, 2021-a). 

Since the rates are so high among young women, the U.S. Preventive Services Task Force (USPSTF) recommends that sexually active women under 25 get screened yearly. Women older than 25 should also be screened if they are at higher risk of infection. For example, those who participate in high-risk sexual behavior such as unprotected sex and sex with multiple partners are at higher risk. So part of the increase in chlamydia cases could be due to more women getting tested (USPSTF, 2021).  

But men are not off the hook—as mentioned, the rates of chlamydial infection in men have gone up almost 39%. This increase may also be due to increased screening and diagnosis, especially in men who have sex with men (MSM). Screening guidelines recommend that MSM get tested at least annually or more often if they are at high risk (USPSTF, 2021).

3. Cuts in STI-prevention programs

Public health programs—never particularly well funded in the U.S.—have been cut even further in recent years (Devi, 2020). It takes money to run sexual health programs like STI prevention. 

According to the CDC, more than half of local programs have undergone budget cuts, leading to closed clinics, fewer opportunities for chlamydia screening, and more difficulty in helping people get the care they need. Lack of funding also hurts education programs, causing a fall in sexual health and STI awareness (CDC, 2019).

How to prevent chlamydia transmission?

The only sure way to avoid chlamydia and STIs is to abstain from sexual contact. The next best way is to use barrier protection, like condoms, during sex. Remember that unprotected sex increases your risk of STIs. Contraceptives like birth control pills, etc., may prevent pregnancy but don't protect against getting an STI.

If you have symptoms of chlamydia, see a healthcare provider as soon as possible for testing. Follow screening recommendations and if you test positive for chlamydia, let your sex partner(s) know so that they can get tested and treated. 


If you have any medical questions or concerns, please talk to your healthcare provider. The articles on Health Guide are underpinned by peer-reviewed research and information drawn from medical societies and governmental agencies. However, they are not a substitute for professional medical advice, diagnosis, or treatment.

Centers for Disease Control and Prevention (CDC). (2021-b, Apr). Reported STDs reach all-time high for 6th consecutive year. Retrieved on May 13, 2021 from

Centers for Disease Control and Prevention (CDC). (2019, Oct) New CDC Report: STDs continue to rise in the U.S. Retrieved on May 13, 2021 from

Centers for Disease Control and Prevention (CDC). (2018, Aug). 2018 STD Prevention Conference. Retrieved on May 13, 2021 from

Hammerschlag, M.R. (2018). Chlamydia trachomatis infections in the newborn. In UptoDate . Weisman, L.E., Morven, S.E., and Armsby, C. (Eds.). Retrieved from

Hsu, K. (2019). Clinical manifestations and diagnosis of Chlamydia trachomatis infections. In UptoDate . Marrazzo, J. and Bloom, A. (Eds.). Retrieved from

Holt, M., Lea, T., Mao, L., Kolstee, J., Zablotska, I., Duck, T., et al. (2018). Community-level changes in condom use and uptake of HIV pre-exposure prophylaxis by gay and bisexual men in Melbourne and Sydney, Australia: results of repeated behavioural surveillance in 2013-17. The Lancet. HIV , 5(8), e448–e456. doi: 10.1016/S2352-3018(18)30072-9. Retrieved from

U.S. Preventive Services Task Force (USPSTF). (2021). Draft recommendation statement: screening for chlamydia and gonorrhea. Retrieved May 13, 2021 from

How we reviewed this article

Every article on Health Guide goes through rigorous fact-checking by our team of medical reviewers. Our reviewers are trained medical professionals who ensure each article contains the most up-to-date information, and that medical details have been correctly interpreted by the writer.

Current version

May 21, 2021

Written by

Michael Martin

Fact checked by

Mike Bohl, MD, MPH, ALM

About the medical reviewer

Dr. Mike is a licensed physician and a former Director, Medical Content & Education at Ro.