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 years are at highest risk for the disease but data from the Centers for Disease Control and Prevention indicate the infection is most common among women from 20 to 25 years of age.
  
  
• Number of sexual partners. The higher the number of sexual partners, the greater the risk of contracting chlamydia. The disease is more common in individuals who are not in long-term, monogamous relationships.
  
  
• Lack of or improper condom use. People who do not use condoms, or who use them inconsistently or improperly 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 an STD face a higher risk of getting chlamydia.
  
  
• A sex partner who has not been treated. People with chlamydia whose sexual partners have not been treated face a higher risk of becoming reinfected. Also, people with a previous history of STDs are at a higher risk of developing chlamydia.