  We analyse the infection-age-dependent SIR model from a numerical point of
view. First, we present an algorithm for calculating the solution the
infection-age-structured SIR model without demography of the background host.
Second, we examine how and under which conditions, the conventional SIR model
(without infection-age) serves as a practical approximation to the
infection-age SIR model. Special emphasis is given on the effective
reproduction number.
