Chlamydia is a sexually transmissible infection that can affect both men and women. It is caused by bacterium. Many people who are infected with the bacterium do not have symptoms of infection but can still transmit the bacterium. Chlamydia can affect the urethra (the urine passage), cervix (the neck of the womb), rectum and anus, throat, and eyes. Chlamydia infection can result in complications, but it is preventable.