where S(t) is the average number of susceptibles at time t, I(t) is the average number of infectious, N is the size of the population, which decreases due to disease-induced deaths (taken as a fraction a of progressing infections), b is the contact rate, and c 21 is the infectious period. After an average residence time c 21 , infectious individuals recover or die (not shown in [1] ). The total number of cases up to time t, T(t) obeys the equation dT/dt = b S/N I. Epidemic reports most commonly state the occurrence of new infected cases, which over the period t, are given by