The bacterium Chlamydia trachomatis causes Chlamydia. The bacterium can enter the body and cause chlamydia infection when the mucous membrane (soft moist tissues of the body), such as the genitals or rectal area, come into contact with mucous membrane secretions, semen, or vaginal fluid of an infected person, primarily during vaginal or anal sex. Oral sex is not a common cause of infection with this bacterium.
Chlamydia is less likely to be transmitted during oral sex because the bacteria that cause chlamydia prefer to target the genital area rather than the throat. This is why it is unlikely for chlamydia to be transmitted from mouth-to-penis and penis-to-mouth contact, although it is still possible. It is even less likely for transmission to take place from vagina-to-mouth or anus-to-mouth contact. Transmission is not known to occur from mouth-to-vagina and mouth to anus contact.
Any time infected secretions or fluids come into contact with a mucous membrane of the body, transmission is possible, even if the penis or tongue does not fully penetrate the vagina or anus. For this reason, women who have never had anal sex can still get chlamydia in the anus or rectum, because the bacteria can be transmitted when wiping secretions with toilet paper. Also, chlamydia in the eye is possible when discharge carries the disease into the eye during sex or hand-to-eye contact with semen or vaginal fluid.
Vaginal delivery of a newborn by an infected mother may also cause chlamydia to occur, which can result in eye infections, pneumonia, or other complications. The Centers for Disease Control and Prevention recommends all pregnant women be screened for chlamydia during the first prenatal exam, and women should be screened again later during the pregnancy if they have a high risk of infection.