Schematic overview of the model framework. The model has a susceptible-infected-susceptible (SIS) framework and allows three hypothetical processes for the timing of progression from chlamydia to PID to be investigated. A woman can become infected at rate λ (force of infection), can clear her infection naturally (rate r), or can be effectively screened and treated (rate α). Numbers indicate when during the chlamydia infection progression to PID could happen: 1) immediate progression, 2) constant progression, and 3) progression at the end of infection. For all three types of progression a certain fraction f of chlamydia-infected women will develop PID. For the constant progression model a woman moves from being infected without PID (I1) to being infected with PID (I2) at rate γ, which is set to . For immediate progression and the progression at the end of infection we set and .