How do people become infected with pid?

By having sex: Pid results from catching a contagious infection, usually through sexual intercourse, but sometime with more "casual" contact. The gonococcus can persist for a while on environmental surfaces, but catching it from "doorknobs" must be exceedingly rare.

Usually due to STD: The large majority of PID cases result from STDs, usually chlamydia or gonorrhea, especially in women under 25-30 years old However, some cases are unrelated to sex, especially if the woman has had prior PID, which can predispose to recurrent, non-STD pelvic infection. You (or the person with PID) should follow doctor's advice about diagnosis, treatment, follow-up, and likely partner treatment.