Figure 1. The Susceptible-Infectious-Recovered-Susceptible (SIRS) model of high-risk Human Papillomavirus (HPV) transmission and potential consequent progression to cervical cancer.

Susceptible individuals become infected at a rate proportional to the force of infection λ. Following infection, they may progress in a stepwise fashion to neoplastic lesions of increasing severity (CIN1, CIN2, CIN3) and then to cancer (with rates ). Alternatively, they may spontaneously clear the infection from any pre-cancerous stage (with rates ). High-grade clinical lesions (CIN2 and CIN3) may be identified by cytological screening and successfully treated (at a rate π). Following viral clearance or successful treatment of lesions, women retain an immunity to re-infection with the same HPV type. This immunity wanes at rate κ, precipitating a return to the Susceptible compartment.