PID part 2. Pid is really an std. All the organisms that cause pid are std's. Therefore, protection against std is the best prevention from acquiring pid.
PID part 1. Younger, sexually active women who have many different sexual partners and do not use barrier protection are at higher risk for pid. Many bacteria are causative, & pid can be asymptomatic or severely painful, mimicking appendicitis. Incidence is not clearly known but guessed to be about 1% of women of child bearing age. The picture is how pid looks inside the pelvis in a very severe case.