Chlamydia Transmission: How It Spreads and Who Is at Risk

Chlamydia trachomatis: The Most Common Bacterial STI

Chlamydia is a sexually transmitted infection caused by the obligate intracellular bacterium Chlamydia trachomatis. It infects columnar epithelial cells of the urogenital tract, rectum, pharynx, and conjunctiva.

Transmission Routes

Vaginal sex (penis-vagina)
Anal sex (penis-anus)
Oral sex (mouth-penis, mouth-vulva, mouth-anus) — less efficient but documented
Shared sex toys without cleaning/condom change between partners
Perinatal: Mother to baby during vaginal delivery → neonatal conjunctivitis, pneumonia

What Does NOT Transmit Chlamydia

Casual contact (hugging, kissing, sharing towels, toilet seats)
Swimming pools, hot tubs
Objects (fomites) — the organism dies quickly outside host cells
Animals (human strains are host-specific; C. psittaci from birds causes psittacosis, not urogenital chlamydia)

Who Can Transmit?

Anyone with the infection—symptomatic or asymptomatic. 70% of women and 50% of men have no symptoms but remain infectious.

Risk Factors

New or multiple sex partners
Inconsistent condom use
Age <25 (highest prevalence)
Prior STI
MSM (men who have sex with men) for rectal/pharyngeal infection

Prevention

Correct, consistent condom use (male/external or female/internal)
Mutual monogamy with tested partner
Regular screening: annually for sexually active women <25; risk-based for others; at first prenatal visit
Expedited partner therapy (EPT) where legal

Treatment

Doxycycline 100 mg BID × 7 days (first-line, non-pregnant)
Azithromycin 1 g single dose (alternative; pregnancy-safe)
Test-of-cure at 3–4 weeks if pregnant, symptomatic persistence, or rectal infection
Abstain until 7 days after treatment completion (both partners)

Untreated chlamydia → PID, infertility, ectopic pregnancy, chronic pelvic pain (women); epididymitis, reactive arthritis (men); neonatal infection.