A man gets chlamydia through sexual contact with an infected partner. The bacteria pass from one person’s body to another during vaginal, anal, or oral sex, even when neither partner has visible symptoms. About half of men with chlamydia never develop any signs of infection, which means many men contract it from partners who genuinely don’t know they’re carrying it.
How the Bacteria Enter the Body
Chlamydia is caused by the bacterium Chlamydia trachomatis, which specifically targets the thin, moist tissue lining the urethra, rectum, and throat. During unprotected sex, the bacteria land on these surfaces and essentially trick cells into absorbing them. Once inside a cell, the bacteria shift into a more active form, multiply over two to three days, and then burst out or push their way free to infect neighboring cells. This cycle repeats, spreading the infection along the tissue.
The urethra (the tube that runs through the penis) is the most common site of infection in men who have vaginal or oral sex. Men who have receptive anal sex can develop a rectal infection, and oral sex can lead to a throat infection, though that’s less common. In all cases, the bacteria need direct contact with mucosal tissue to establish an infection. You can’t get chlamydia from a toilet seat, a swimming pool, or casual contact like hugging.
Transmission Risk Per Encounter
Not every sexual encounter with an infected partner results in transmission. Research published in the journal Sexually Transmitted Infections estimated the per-act transmission probability at roughly 4.5% for a single unprotected episode. That number might sound low, but risk accumulates quickly over multiple encounters with the same partner. In an ongoing relationship where one person is infected and the couple isn’t using protection, transmission becomes likely within weeks.
Condoms, when used correctly and consistently, reduce the risk of chlamydia by more than 90%. That protection drops significantly with inconsistent use or improper technique (putting the condom on partway through sex, for example). Condoms also don’t cover all potentially exposed tissue during oral or anal contact, so risk isn’t eliminated entirely.
Why Many Men Don’t Realize They’re Infected
Roughly 50% of men with chlamydia experience no symptoms at all. Among those who do, symptoms typically don’t appear until several weeks after exposure. This delay creates a window where a man can unknowingly pass the infection to new partners.
When symptoms do show up, they usually include:
- Burning or pain during urination
- A clear or cloudy discharge from the penis
- Redness, swelling, or itching at the tip of the penis
- Swelling or tenderness in one or both testicles (less common, and typically a sign the infection has spread)
Rectal infections may cause discharge, pain, or bleeding, while throat infections are often completely silent. Because symptoms are unreliable, testing is the only way to know for sure.
How Testing Works
The standard test for chlamydia is a nucleic acid amplification test, or NAAT, which detects the bacteria’s genetic material. For men, this usually means providing a urine sample. If there’s a potential rectal or throat infection, a clinician may use a swab at those sites instead. Results typically come back within a few days. NAAT has largely replaced older culture-based methods because it’s faster and more accurate.
Because of the incubation period, testing too soon after a possible exposure can produce a false negative. Most guidelines recommend waiting at least two weeks after the encounter in question before getting tested, though some clinics suggest testing and then retesting if the first result is negative and risk was high.
Reinfection and the Ping-Pong Effect
Getting treated for chlamydia doesn’t make you immune to it. Studies estimate that about 10% of heterosexual men who’ve been treated for chlamydia get reinfected within a year. The most common reason is straightforward: their sexual partner was never tested or treated, so the bacteria pass back and forth between them.
Research in The Journal of Infectious Diseases found that chlamydia infections actually clear more slowly in men than in women, but are less likely to take hold in the first place. This means that when a man does get infected, the bacteria can linger longer, creating an ongoing reservoir that reinfects a treated partner. The researchers specifically recommended stronger focus on notifying and treating male partners as a critical step in breaking this cycle. If you’re diagnosed, your recent partners need to know so they can get tested and treated before anyone passes the infection back.
Who Faces the Highest Risk
Any sexually active man can get chlamydia, but certain factors raise the odds. Having multiple partners increases cumulative exposure. Men under 25 have higher infection rates across the board. Men who have sex with men face additional risk because rectal tissue is especially vulnerable to the bacteria, and rectal infections often produce no symptoms.
A previous chlamydia infection is itself a risk factor, not because it weakens your defenses, but because it signals ongoing exposure patterns that haven’t changed. If you’ve had chlamydia before, periodic screening (at least once a year, or more often with new partners) catches reinfections early.
What Happens If It Goes Untreated
Most conversations about chlamydia complications focus on women, where untreated infections can cause infertility. Men face fewer long-term consequences, but they’re not zero. The infection can spread from the urethra to the epididymis, the coiled tube behind each testicle that stores sperm. This causes pain and swelling and, in rare cases, can affect fertility. Untreated rectal chlamydia increases vulnerability to other sexually transmitted infections, including HIV, because the inflamed tissue provides an easier entry point for other pathogens.
Treatment itself is simple: a short course of antibiotics clears the infection in the vast majority of cases. Retesting three months after treatment confirms the infection is gone and catches any reinfection early.