The basic reproduction ratio for a model of directly transmitted infections considering the virus charge and the immunological response

in order to describe mathematically the transmission of microparasites, esp ecially directly transmitted infections, it is usual to set up differential equations assuming the mass action law and a homogeneously mixed populatio n. In this paper we analyze such a model taking into account heterogeneity with respect to the infectivity, that is, the variability in the evolution of the interaction between parasite and the human host during the infectiou s period. The well established biological phenomenon of initial increase in parasite abundance followed by its decrease, due to the interaction betwee n the host's immunological response and the parasite, has thus been taken i nto account. The variable amount of microparasites eliminated by an infecti ous individual, and the different (heterogeneous) immunological response bu ild up by the host when in interaction with parasite are present in the mod el. The analytical expression for the basic reproduction ratio is derived t hrough stability analysis.