PID is infection and swelling inside a women’s reproductive organs, sometimes caused by an STD. Untreated chlamydia or gonorrhea causes PID, so if you think you may have an STD, or be at risk, see your health care provider right away. Women who have had chlamydia more than once are more likely to get PID. Many times women are reinfected with chlamydia because their partner doesn’t get treated. Younger women are more at risk for PID. You are at risk for PID if you’ve had a lot of sex partners or your partner has many other sex partners.