Chlamydia spreads through sexual contact with an infected person. It can be transmitted during vaginal, anal, or oral sex, and it doesn’t require ejaculation to pass from one person to another. With over 1.5 million cases reported in the United States in 2024 alone, it remains the most commonly reported bacterial sexually transmitted infection in the country.
Sexual Contact Is the Primary Route
The bacterium that causes chlamydia, Chlamydia trachomatis, passes between people through infected genital secretions. Any unprotected sexual contact that involves the exchange of these fluids can transmit the infection. Vaginal and anal sex carry the highest risk, but oral sex is also a confirmed route. You can get chlamydia in your throat from performing oral sex on an infected partner, and an infection in someone’s throat can spread to a partner’s genitals during oral sex as well.
Chlamydia can infect the urethra, cervix, rectum, and throat. Where you get infected depends on the type of sexual contact. Someone who has receptive anal sex, for example, can develop a rectal infection even if they don’t have a genital one. These site-specific infections matter because standard urine tests won’t catch a throat or rectal infection, so testing needs to match the type of sex you’re having.
You Can’t Get It From Toilet Seats or Casual Contact
The chlamydia bacterium does not survive outside the human body. You cannot catch it from toilet seats, swimming pools, shared towels, or hugging and kissing. It requires direct contact with infected mucous membranes or secretions to spread. This is a common source of confusion, but the biology is clear: the organism depends entirely on living human cells to survive and reproduce.
Most People Who Have It Don’t Know
The reason chlamydia spreads so effectively is that most carriers have no symptoms at all. About 75% of women and 50% of men with chlamydia experience no noticeable signs of infection. They feel completely fine and have no reason to suspect they’re carrying the bacterium, yet they can still pass it to sexual partners.
When symptoms do appear, they typically show up several weeks after exposure. In women, symptoms can include unusual vaginal discharge, burning during urination, or bleeding between periods. Men may notice discharge from the penis, burning with urination, or pain and swelling in one testicle. Rectal infections can cause discharge, pain, or bleeding regardless of sex. Throat infections are often completely silent.
Because the window between exposure and symptoms is long, and because most people never develop symptoms, you can carry and spread chlamydia for months without knowing. This is why routine screening matters far more than waiting for something to feel wrong.
Transmission During Childbirth
A pregnant person with an untreated chlamydia infection can pass the bacterium to their baby during vaginal delivery. As the baby moves through the birth canal, it comes into contact with infected tissue. This can cause eye infections (conjunctivitis) or pneumonia in the newborn. This is one reason chlamydia screening is recommended for all pregnant people under 25 and for those 25 and older who have risk factors.
Why the Infection Takes Hold So Easily
Chlamydia is unusually well adapted to infecting human cells. The bacterium exists in two forms: a tiny infectious particle (roughly 0.3 micrometers) that can survive briefly between hosts, and a larger form that replicates inside your cells. When the infectious particle contacts a cell lining your cervix, urethra, rectum, or throat, it’s pulled inside within minutes. Once there, it creates a protected pocket within the cell and begins hijacking the cell’s own nutrients and energy to multiply. Within 8 to 12 hours it transforms into its replicating form, and over the next 24 to 48 hours it divides repeatedly before bursting out to infect neighboring cells.
This cycle of invasion and replication is why chlamydia can cause real tissue damage over time, particularly in the reproductive tract, even when you feel nothing.
Who Gets Tested and How Often
Because chlamydia is so often invisible, the CDC recommends routine screening based on age, sex, and sexual behavior rather than symptoms. The current guidelines break down like this:
- Women and people with a cervix under 25: Annual screening if sexually active.
- Women and people with a cervix 25 and older: Annual screening if you have risk factors like a new partner, multiple partners, a partner with other partners, inconsistent condom use, or a previous STI.
- Pregnant people: Screened at the first prenatal visit, with repeat testing in the third trimester for those under 25 or at increased risk.
- Men who have sex with men: At least annually at all sites of contact (urethra, rectum), regardless of condom use. Every 3 to 6 months if on PrEP, living with HIV, or if either partner has multiple partners.
- Heterosexual men: Routine screening isn’t broadly recommended, but it’s worth considering in high-prevalence settings like STI clinics or correctional facilities.
If you test positive and complete treatment, you should be retested about three months later. Reinfection is common, often because a partner wasn’t treated at the same time.
How to Reduce Your Risk
Latex condoms, used consistently and correctly, reduce the risk of chlamydia transmission by acting as a physical barrier to the genital secretions that carry the bacterium. They’re not perfect, particularly because chlamydia can infect areas a condom doesn’t cover, but epidemiological studies consistently show lower infection rates among people who use them.
Beyond condoms, limiting your number of sexual partners and ensuring that both you and your partners get tested regularly are the most practical ways to lower your risk. If you’re diagnosed, your recent sexual partners need to be notified and treated. Otherwise, you’re likely to get reinfected the next time you have sex with them, even after completing your own treatment.
Chlamydia is curable with a short course of antibiotics, and the earlier it’s caught, the less chance it has to cause complications like pelvic inflammatory disease, chronic pain, or fertility problems. The challenge isn’t treatment. It’s detection. Given that three out of four women and half of men with chlamydia have no idea they’re infected, the most reliable protection is regular screening paired with consistent condom use.