Another epidemic model due to Cooke et alia [3] that is
more complicated than Problem 4 involves the size I(t) = y_{1}(t)
of an infected population and the total size N(t) = y_{2}(t) of
the population at time t. The equations are

As in Problem 2, it is convenient to pass the parameters to the function for evaluating the DDEs as global variables or to hard code them. In [3] the solution is investigated for a number of l, so pass it as a parameter through dde23. Values l = 12, 15, 20, 28 are of interest. You might find it interesting to compare your plots to those of Figure 4 in [3]. The following figure shows the case l = 12 .