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 the reproductive organs.

The disease affects both women and men, although reported cases for women greatly exceed those for men. Chlamydia affects people in all age groups, but it is particularly prevalent among teenagers and young adults. According to the U.S. Centers for Disease Control and Prevention (CDC), the infection is most prevalent among women aged 20 to 25.

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.

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 is 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.

Chlamydia can also be passed from an infected mother to her infant during vaginal childbirth. Transmission to the newborn results from exposure to the mother’s infected cervix during birth. Infants with chlamydia may be born prematurely or have conjunctivitis (an infection of the eye, also known as “pink eye”) as well as pneumonia. Chlamydia is one of the most common STDs reported by physicians and other health providers to the CDC.

Estimates of the exact number of chlamydia cases vary. In 2004, there were 929,462 chlamydial infections 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.