PID occurs when bacteria move upward from a woman's vagina or cervix (opening to the uterus) into her reproductive organs. Many different organisms can cause PID, but many cases are associated with gonorrhea and chlamydia, two very common bacterial STDs.
A prior episode of PID increases the risk of another episode because the reproductive organs may be damaged during the first infection. Sexually active women in their childbearing years are most at risk, and those under age 25 are more likely to develop PID than those older than 25. This is partly because the cervix of teenage girls and young women is not fully matured, increasing their susceptibility to the STDs that are linked to PID. The more sex partners a woman has, the greater her risk of developing PID and a woman whose partner has more than one sex partner is at even greater risk of developing PID.
Women who douche may have a higher risk of developing PID compared with women who do not douche. Research has shown that douching changes the vaginal flora (organisms that live in the vagina) in harmful ways, and can also force bacteria into the upper reproductive organs from the vagina.