  Epidemiological data on seasonal influenza show that the growth rate of the
number of infected individuals can increase passing from one exponential growth
rate to another one with a larger exponent. Such behavior is not described by
conventional epidemiological models. In this work an immuno-epidemiological
model is proposed in order to describe this two-stage growth. It takes into
account that the growth in the number of infected individuals increases the
initial viral load and provides a passage from the first stage of epidemic
where only people with weak immune response are infected to the second stage
where people with strong immune response are also infected. This scenario may
be viewed as an increase of the effective number of susceptible increasing the
effective growth rate of infected.
