Chlamydia is caused by the bacteria Chlamydia trachomatis, which is found in vaginal fluids and semen. Chlamydia trachomatis can be passed through vaginal, anal or oral sex.

There are a number of risk factors for chlamydia. They are:

• Age. Because the cervix of teenage girls and young women is not fully matured and more susceptible to infection, this population faces a particularly high risk for infection if sexually active. Women and adolescents through age 20 are at highest risk for the disease. But data from the Preventive Services Task Force (PSTF) indicate the infection is most common among U.S. women aged 20 to 25 years.
  
  
• Number of sexual partners. The higher the number of sexual partners, the greater the risk of contracting chlamydia. The disease is more common in unmarried women due the absence of a long-term, single-partner relationship. One study showed that unmarried women ages 15 to 34 were seven times more likely to be infected than married women in the same age group.
  
  
• Lack of or improper condom use. People who do not use condoms or use them inconsistently during sexual intercourse are at a greater risk of becoming infected.
  
  
• A sex partner with a sexually transmitted disease (STD). People with a sexual partner with a sexually transmitted disease face a higher risk of getting chlamydia. Also, if the woman or her partner has a previous history of STDs, they are at a higher risk of developing chlamydia.
  
  
• A sex partner who has not been treated. Women with chlamydia whose sexual partners have not been treated face a higher risk of becoming re-infected. A few studies have examined the use of "partner packs" in treatment. In these studies, infected women were given antibiotics for their sexual partners. While this reduced the rate of infection, there were questions of cost and problems with administering antibiotics to untested partners. This is not considered standard practice today, but is still under consideration.