Overall Population Patterns in the Transmission Cycle of Infectious Disease Agents

The present paper presents new models for the transmission of microparasites and macroparasites, which take into account the change of the age structure of the host population due to parasite-induced mortality. The equilibrium density or prevalence of parasites is positive if and only if the basic reproductive rate, R, of the parasites is greater than 1. It is suggested that this condition is not sufficient to determine the critical community size, N*, required to maintain an endemic parasite population. Formulas will be derived which will allow us to estimate N* on the basis of R and on the duration of the immunity induced by an infection.