Fig. 1

A flow diagram of the SIAIAD trachoma infection transmission model. Individuals begin as susceptible to their first infection (S), become infected at a rate λ, progress to the incubating class (I) and leave at a rate γ _i, which does not depend on the previous number of infections they have had. They progress into the infected with active disease compartment (AI) and leave at a rate σ _i, which does depend on the previous number of infections experienced. They lastly progress into the active disease compartment (AD), where they are not infectious but still have identifiable active disease, individuals recover from active disease and become susceptible to infection again at a rate ζ _i, which depends on the number of infections experienced