Schematic overview of the model framework. The Susceptible-Infected-Susceptible (SIS) compartmental model allows investigating three hypothetical temporal relation assumptions between chlamydia infection and PID. A susceptible woman (S) can become infected (I) at a constant rate λ and can clear the infection naturally at rate r. Numbers indicate when during the chlamydia infection progression to PID could happen: 1) immediate progression, 2) constant progression, and 3) progression at the end. For the constant progression, an infected woman can progress at rate γ from being infected without PID (I1) to being infected with PID (I2). We set γ = 0 and I = I1 + I2 for immediate progression and progression at the end. PID, pelvic inflammatory disease.