The self-sustaining chain reaction that constitutes a major epidemic is possible only if R 0 is greater than one. If it is less than one, although there may be some self-limiting chains of secondary transmission following the introduction of an index case (and quite large clusters become possible as R 0 approaches one), this will not lead to a sustained increase in cases and, in a large population, only a small proportion of susceptible hosts will be infected [18] . An important related number is the net (or effective) reproduction number, R t . This is defined as the average number of secondary cases generated by a case infected at time t, accounting for incomplete host susceptibility to infection and control measures in place. If R t is greater than one at time t, the epidemic will (on average) be growing. If R t is less than one, it will be declining [16, 19] .