Chlamydia can be spread during vaginal , anal , or oral sex , and can be passed from an infected mother to her baby during childbirth. Prevention is by not having sex , the use of condoms , or having sex with only one other person, who is not infected. Chlamydia is one of the most common sexually transmitted infections, affecting about 4. The infection can be passed through vaginal, anal, or oral sex. PID can cause scarring inside the reproductive organs , which can later cause serious complications, including chronic pelvic pain, difficulty becoming pregnant , ectopic tubal pregnancy , and other dangerous complications of pregnancy.