Chlamydia is the most common bacterial STI in the U.S. — here’s what you need to know

A close-up of chlamydia that looks like swirls of purple on a white background
Chlamydia is the most common STI caused by bacteria, but because it's typically asymptomatic, many people can have it and not know it. (Photo: Getty Images)

There are more than 20 types of sexually transmitted infections (STIs), but some are more prevalent than others — namely, chlamydia, which is the most common STI caused by bacteria. (HPV, also known as human papillomavirus, is the most common STI caused by a virus.)

In fact, chlamydia trachomatis — the bacteria behind chlamydia infections — is “responsible for the greatest number of sexually transmitted infections” worldwide. In 2020, nearly 1.6 million cases of chlamydia were reported in the U.S, according to the Centers for Disease Control and Prevention (CDC), “making it the most common notifiable sexually transmitted infection in the United States for that year.”

As Dr. Saahir Khan, infectious disease expert with Keck Medicine of USC, tells Yahoo Life: “Chlamydia has been infecting humans for millennia and has evolved to be highly infectious with sexual exposure.”

Here’s what everyone needs to know about chlamydia, according to experts.

You could have chlamydia and not know it

Why is chlamydia so widespread, beating out other common bacterial STIs including gonorrhea? In a nutshell: it’s stealth.

The vast majority of chlamydia cases are asymptomatic, which means “you can have an infection and have no signs or symptoms and you can pass that on to your sex partners,” Dr. Matthew Hamill, assistant professor of medicine specializing in STIs at Johns Hopkins Medicine, tells Yahoo Life. He adds: “It’s the asymptomatic nature that facilitates its spread.”

A lack of screening — testing for chlamydia before the onset of symptoms — is another factor. “Unless you do screenings, then you miss a whole bunch and you only really diagnose the small proportion who develop symptoms,” says Hamill.

Adding to the mix is the fact that some healthcare providers are “really bad at talking about sex with patients,” he says. “It’s not always part of the consultation, and people find it awkward and don’t know what to say — and the opportunity to test is missed.”

In some cases, men and women do experience symptoms

While most chlamydia infections are asymptomatic, some may experience symptoms a few weeks after infection that include “pain with urination or sexual intercourse, cloudy drainage in urine or genital secretions or redness, swelling or ulceration(s) around their genitals,” says Khan.

Hamill says that the most common symptoms men experience is clear discharge coming from the tip of the penis, as well as burning or stinging when urinating. In rare cases, “men can also get pain in the testicles because the infection can spread,” he adds.

For women, symptoms can include abnormal vaginal discharge that’s yellowish or gray and has a strong odor, more frequent urination, pelvic pain and spotting (a small amount of blood) in between periods or after sex.

Chlamydia is spread through sexual contact

You can get chlamydia through unprotected vaginal, anal or oral sex with an infected person or by sharing sex toys with someone who is infected. It’s spread through semen or vaginal fluid that contains the bacteria. Pregnant people who are infected can also pass chlamydia to their babies in the birth canal, which can cause infants to develop a type of eye inflammation known as conjunctivitis and/or pneumonia.

You can’t, however, get chlamydia from kissing, using a toilet after another person or sharing food and drinks, according to the Cleveland Clinic.

Safer sex practices can help people avoid getting chlamydia. “Both male and female condoms work well to prevent chlamydia,” says Hamill. “Most don’t use condoms for oral sex and that’s a way chlamydia can be successfully transmitted.” He adds that the key is to use condoms correctly, “consistently and throughout the sex act.”

Certain age groups are more likely to get chlamydia

“Any age group can get chlamydia,” says Khan, “but it is most common in younger adults with multiple sexual partners.” In 2020, more than 60% of all reported chlamydia cases were among teens and young adults 15 to 24 years old, according to the CDC.

Screenings are the best way to find out if you have the STI. The CDC recommends chlamydia screenings for sexually active women under age 25, all pregnant women under 25 and men who have sex with men. “The CDC guidelines do not recommend screening for men except for very specific circumstances,” says Hamill, who notes that may be because the “potential harm in terms of reproductive health outcomes is much more severe in women.”

Along with getting screened at a doctor’s office or health clinic, there are also at-home STI screening kits, such as I Want the Kit, which is free, tests for chlamydia and gonorrhea and is available to those who live in Maryland, Oklahoma, Alaska or Arizona. At-home test kits “can help with STI control and empower individuals so they feel they're in charge of their sexual health,” says Hamill.

Chlamydia is curable

The STI is treatable with antibiotics. “The standard treatments are azithromycin given as a single dose or three-day course, or doxycycline given as a seven-day course,” says Khan. Hamill adds that doxycycline has become the standard treatment since it’s “more efficacious than azithromycin.”

But, Hamill says, the treatment doesn’t work if someone doesn’t take all of the medication or if they have sex with another (or the same) infected person. “It’s like a game of ping-pong but it’s really serious, going back and forth between partners,” he says. So it’s important that any other sex partners also get treated.

Not treating chlamydia can have serious health consequences

If left untreated, chlamydia can affect fertility and lead to pelvic inflammatory disease, which causes symptoms such as lower abdominal pain and tenderness, fever, painful sex and irregular periods, according to the CDC. Pelvic inflammatory disease has its own complications, including ectopic pregnancy and the formation of scar tissue that can block the fallopian tubes, according to the CDC.

“In severe cases, it can even lead to sterility,” says Khan.

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