Because its two most common causes are both sexually transmitted, sexual activity is by far the greatest single risk factor for PID. Youth also increases the odds; roughly 75 percent of all cases of PID occur in sexually active women under 25 years of age. For reasons not yet understood, younger women are more susceptible to chlamydial and gonorrhea-associated infection than older women are. Also, the disease rarely occurs in nonmenstruating women such as girls who have not yet reached sexual maturity as well as pregnant and postmenopausal women.

Clearly, there is a direct relationship between the number of sexual partners a woman has and the risk of PID—the more partners, the greater the risk. A woman who has only one partner does not have an increased risk. If he has had a vasectomy, the risk of PID is actually lowered.