Flow diagram of the infection models for HPV (A) and chlamydia (B).

HPV virions, V, only infect uninfected basal cells, U_b, to become basal infected cells, I_b. Since HPV is non-lytic, infected cells follow the typical epithelial life-cycle up to the surface passing through different life stages (parbasal I_p, differentiated I_d, differentiated at the surface I_s). Model 3. In the case of C. trachomatis, the elementary bodies, EBs, start the infection by infecting uninfected cells in the upper layers (β_u U_d E_u creates I_d). The EB populations start in the upper layers, E_u, and then migrate down, η_u, into the lower layers, E_l. As EBs migrate down layers they enter uninfected cells (U_b and U_p) and create infected cells (I_b and I_p) which die at rate α (boxes with square represent dead cells). The host immune response, A, is activated by infected basal cells in the case of HPV and all EBs in the case of C. trachomatis. Model 4. Note that for wart-associated HPV infections ρ_a = 0 and α_b = α_p = 1. See Table 3 for parameter descriptions and estimates.