Chlamydia is a bacterial infection of the genital tract spread through sexual contact. If untreated, it can result in various medical complications, including damage to a woman’s reproductive organs.

The disease affects both women and men, though reported cases for women greatly exceed those for men. Although it affects people in all age groups, it is particularly prevalent among teenagers and young adults.

Chlamydia is sometimes confused with gonorrhea, another sexually transmitted disease (STD) caused by a different type of bacteria. Gonorrhea and chlamydia have similar symptoms and, if untreated, can have similar complications. Both are treated with antibiotics, but a different type of antibiotic is used to treat each disease.

Chlamydia is a type of vaginitis, a disorder that causes swelling or infection of the vagina and vulva. In women, the bacteria that causes chlamydia often infects the cells of the cervix, but can spread to the uterus, fallopian tubes and ovaries. In men, the bacteria affect the urethra (the tube that carries urine from the bladder to outside the body), but can spread to the epididymis (the tube that carries the sperm from the testicles).

Chlamydia can be transmitted through vaginal, oral or anal sex. Since chlamydia can be transmitted by anal sex, women or men who have anal intercourse can become infected in the rectum (the last several inches of the intestines that ends at the anus). Although it is less common, chlamydia can also infect the throats of women and men who have oral sex with an infected partner

According to the National Women’s Health Information Center, there is no evidence that chlamydia is sexually transmitted between women who are sexually active with other women. However, large-scale studies have not been conducted to provide reliable information in this area. According to the Preventive Services Task Force (PSTF), the infection is most prevalent among U.S. women aged 20 to 25 years.

Pregnant women who have chlamydia can pass the infection to their babies during vaginal childbirth. Transmission to the newborn results from exposure to the mother’s infected cervix during birth. Infants who are exposed to the chlamydia bacteria may be born prematurely or develop neonatal conjunctivitis (a severe eye infection). Conjunctivitis in newborns can lead to serious complications including pneumonia (inflammation of the lungs), as drainage from the eye enters the nasal passages and infects the respiratory tract. Because of its potential severity, babies born to women with chlamydia are usually given medication in the form of a topical ointment that is applied to their eyes at the time of delivery. 

Chlamydia is the most common STD reported by physicians and other healthcare providers to the Centers for Disease Control and Prevention (CDC), with gonorrhea and syphilis second and third in frequency. However, there are other common STDs that physicians are not required to report to the CDC, such as the human papillomavirus (HPV) and genital herpes.

Estimates for the exact number of chlamydia cases vary. In 2006, more than 1 million cases of chlamydial infections were reported to the CDC. However, since many people are not aware of their infections and are not tested, health officials estimate that the actual number of infections is much higher. Statistics vary among individual health organizations, but most estimates fall within the range of 2.8 million to 4 million chlamydia infections per year. 

Because chlamydia can lead to serious medical problems, there has been increased attention and screening for the disease in recent years. According to the CDC, from 1987 through 2006, the reported rate of chlamydial infection in women increased from 50.8 cases to 347.8 cases per 100,000 people. The jump in reported figures is probably the result of increased screening, use of more sensitive diagnostic tests and improved reporting, as well as the continued high incidence of the disease.